ANDROID: userfaultfd: Fix use-after-free in userfaultfd_using_sigbus()

In 582c6d18 ("ANDROID: userfaultfd: allow SPF for
UFFD_FEATURE_SIGBUS on private+anon"), we allowed userfaultfd
registered VMAs using SIGBUS to be handled with SPF. But during
page-fault handling, before userfaultfd_ctx is dereferenced,
another thread may call userfaultfd_release(), unlink the VMA
and then deallocate the same userfaultfd_ctx, leaving a dangling
pointer behind for dereference.

It is insufficient to do the access under rcu read-lock as the context
may have been deallocated before entering the critical section. Checking
vma has not changed in the critical section ensures we are not looking at
dangling pointer to userfaultfd_ctx.

Change-Id: I9c3ba0f1352e49f0ea387b92c18b5f1b5dcad7f1
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Bug: 349936398