CLONE_EMPTY_MNTNS is (1ULL << 37) = 0x2000000000ULL, not 0x400000000ULL.

Fixes: 5b8ffd63fbd9 ("selftests/filesystems: add clone3 tests for empty mount namespaces")
Signed-off-by: Christian Brauner <brauner@kernel.org>

empty_mntns.h includes ../statmount/statmount.h which provides a
4-argument statmount_alloc(mnt_id, mnt_ns_id, mask, flags), but then
redefines its own 3-argument version without the flags parameter. This
causes a build failure due to conflicting types.

Remove the duplicate definition from empty_mntns.h and update all
callers to pass 0 for the flags argument.

Fixes: 32f54f2bbccf ("selftests/filesystems: add tests for empty mount namespaces")
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a test suite for the CLONE_EMPTY_MNTNS flag exercising the empty
mount namespace functionality through the clone3() syscall.

The clone3() code path is distinct from the unshare() path already
tested in empty_mntns_test.c.  With clone3(), CLONE_EMPTY_MNTNS
(0x400000000ULL) is a 64-bit flag that implies CLONE_NEWNS.  The
implication happens in kernel_clone() before copy_process(), unlike
unshare() where it goes through UNSHARE_EMPTY_MNTNS to
CLONE_EMPTY_MNTNS conversion in unshare_nsproxy_namespaces().

The tests cover:

- basic functionality: clone3 child gets empty mount namespace with
  exactly one mount, root and cwd point to the same mount
- CLONE_NEWNS implication: CLONE_EMPTY_MNTNS works without explicit
  CLONE_NEWNS, also works with redundant CLONE_NEWNS
- flag interactions: combines correctly with CLONE_NEWUSER,
  CLONE_NEWPID, CLONE_NEWUTS, CLONE_NEWIPC, CLONE_PIDFD
- mutual exclusion: CLONE_EMPTY_MNTNS | CLONE_FS returns EINVAL
  because the implied CLONE_NEWNS conflicts with CLONE_FS
- error paths: EPERM without capabilities, unknown 64-bit flags
  rejected
- parent isolation: parent mount namespace is unchanged after clone
- many parent mounts: child still gets exactly one mount
- mount properties: root mount is nullfs, is its own parent, is the
  only listmount entry
- overmount workflow: child can mount tmpfs over nullfs root to build
  a writable filesystem from scratch
- repeated clone3: each child gets a distinct mount namespace
- setns: parent can join child's empty mount namespace via setns()
- regression: plain CLONE_NEWNS via clone3 still copies the full
  mount tree

Link: https://patch.msgid.link/20260306-work-empty-mntns-consolidated-v1-3-6eb30529bbb0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a test suite for the UNSHARE_EMPTY_MNTNS and CLONE_EMPTY_MNTNS
flags exercising the empty mount namespace functionality through the
kselftest harness.

The tests cover:

- basic functionality: unshare succeeds, exactly one mount exists in
  the new namespace, root and cwd point to the same mount
- flag interactions: UNSHARE_EMPTY_MNTNS works standalone without
  explicit CLONE_NEWNS, combines correctly with CLONE_NEWUSER and
  other namespace flags (CLONE_NEWUTS, CLONE_NEWIPC)
- edge cases: EPERM without capabilities, works from a user namespace,
  many source mounts still result in one mount, cwd on a different
  mount gets reset to root
- error paths: invalid flags return EINVAL
- regression: plain CLONE_NEWNS still copies the full mount tree,
  other namespace unshares are unaffected
- mount properties: the root mount has the expected statmount
  properties, is its own parent, and is the only entry returned by
  listmount
- repeated unshare: consecutive UNSHARE_EMPTY_MNTNS calls each
  produce a new namespace with a distinct mount ID
- overmount workflow: verifies the intended usage pattern of creating
  an empty mount namespace with a nullfs root and then mounting tmpfs
  over it to build a writable filesystem from scratch

Link: https://patch.msgid.link/20260306-work-empty-mntns-consolidated-v1-2-6eb30529bbb0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>