UPSTREAM: mm: fix some minor per-VMA lock issues in userfaultfd

Rename lock_vma() to uffd_lock_vma() because it really is uffd specific.
Remove comment referencing unlock_vma() which doesn't exist.  Fix the
comment about lock_vma_under_rcu() which I just made incorrect.

Link: https://lkml.kernel.org/r/20240426144506.1290619-4-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 73b4a0cd)

Bug: 161210518
Change-Id: Ia367f24963b424d3f8108ee3176d2da28b31698d
Signed-off-by: T.J. Mercier <tjmercier@google.com>