selftests/filesystems: add tests for empty mount namespaces

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>

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