userfaultfd: move userfaultfd_ctx struct to header file

Patch series "per-vma locks in userfaultfd", v7.

Performing userfaultfd operations (like copy/move etc.) in critical
section of mmap_lock (read-mode) causes significant contention on the lock
when operations requiring the lock in write-mode are taking place
concurrently.  We can use per-vma locks instead to significantly reduce
the contention issue.

Android runtime's Garbage Collector uses userfaultfd for concurrent
compaction.  mmap-lock contention during compaction potentially causes
jittery experience for the user.  During one such reproducible scenario,
we observed the following improvements with this patch-set:

- Wall clock time of compaction phase came down from ~3s to <500ms
- Uninterruptible sleep time (across all threads in the process) was
  ~10ms (none in mmap_lock) during compaction, instead of >20s


This patch (of 4):

Move the struct to userfaultfd_k.h to be accessible from mm/userfaultfd.c.
There are no other changes in the struct.

This is required to prepare for using per-vma locks in userfaultfd
operations.

Link: https://lkml.kernel.org/r/20240215182756.3448972-1-lokeshgidra@google.com
Link: https://lkml.kernel.org/r/20240215182756.3448972-2-lokeshgidra@google.com
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tim Murray <timmurray@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>