BACKPORT: mm: do_wp_page() simplification

How about we just make sure we're the only possible valid user fo the
page before we bother to reuse it?

Simplify, simplify, simplify.

And get rid of the nasty serialization on the page lock at the same time.

[peterx: add subject prefix]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 09854ba9)

[Kalesh Singh: Resolve conflict in mm/memory.c]
Bug: 176847924
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Change-Id: I0b855b305332564670b8cbea9660405fed89a044