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.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[peterx: add subject prefix]
Signed-off-by: Peter Xu <peterx@redhat.com>
Git-commit: 09854ba94c6aad7886996bfbee2530b3d8a7f4f4
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


[zhenhuah@codeaurora.org: resolve trivial merge conflicts]
Change-Id: I73dc102cb02d751d497bdf3cc39179a4bfdc5a54
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>