userfaultfd: add UFFDIO_REGISTER_MODE_RWP and UFFDIO_RWPROTECT plumbing

Add the userspace interface for read-write protection tracking:

  - UFFDIO_REGISTER_MODE_RWP      register a range for RWP tracking
  - UFFD_FEATURE_RWP              capability bit
  - UFFDIO_RWPROTECT              install / remove RWP on a range

Introduce CONFIG_USERFAULTFD_RWP, auto-selected on 64-bit kernels with
ARCH_HAS_PTE_PROTNONE and HAVE_ARCH_USERFAULTFD_WP.  The symbol gates
VM_UFFD_RWP (previously aliased to VM_NONE) and the smaps/trace-flag hooks
added in the preparatory patches; without it the UAPI bits added here have
nothing to drive and would be unreachable.

Registration sets VM_UFFD_RWP on the VMA.  Combining MODE_WP with MODE_RWP
is rejected because both modes claim the uffd PTE bit.

UFFDIO_RWPROTECT is the bidirectional counterpart of
UFFDIO_WRITEPROTECT:

  - MODE_RWP              change_protection() with MM_CP_UFFD_RWP
                          installs PAGE_NONE and sets the uffd bit on
                          present PTEs
  - !MODE_RWP             change_protection() with MM_CP_UFFD_RWP_RESOLVE
                          restores vma->vm_page_prot and clears the bit

userfaultfd_clear_vma() runs the same resolve pass on unregister so RWP
state cannot outlive the uffd.

Re-registering a range must not drop a mode that installs per-PTE markers
(WP or RWP); doing so returns -EBUSY.  This also closes a pre-existing
window where re-registering without MODE_WP would strand uffd-wp markers:
before, those caused extra write-faults but were otherwise benign; with
RWP preservation in place, a subsequent mprotect() on a VM_UFFD_RWP VMA
would silently promote the stale markers to RWP.

The feature is not yet advertised.  UFFDIO_REGISTER_MODE_RWP,
UFFD_FEATURE_RWP, and _UFFDIO_RWPROTECT are intentionally absent from
UFFD_API_REGISTER_MODES, UFFD_API_FEATURES, and UFFD_API_RANGE_IOCTLS, so
UFFDIO_API masks them out and the register-mode validator rejects the bit.
The follow-up patch adds fault dispatch and exposes the UAPI.

Link: https://lore.kernel.org/20260708111417.173443-10-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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