Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments

Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments

This patch differs from the upstream commit
de33c8db5910cda599899dd431cc30d7c1018cbf written by Linus, as it aims to
only prevent the oops from happening, not attempt to change anything
else.

The problem was introduced by commit
ba470de43188cdbff795b5da43a1474523c6c2fb

which added new references to *vma after we've potentially freed it.

From: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Maksim Yevmenkin <maksim.yevmenkin@gmail.com>
Tested-by: Maksim Yevmenkin <maksim.yevmenkin@gmail.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>