fs: deprecate memclear_highpage_flush

Now that all the in-tree users are converted over to zero_user_page(),
deprecate the old memclear_highpage_flush() call.

Signed-off-by: Nate Diller <nate.diller@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>