mm/page_ref: use atomic_set_release in page_ref_unfreeze

mm/page_ref: use atomic_set_release in page_ref_unfreeze

page_ref_unfreeze() has exactly that semantic.  No functional changes:
just minus one barrier and proper handling of PPro errata.

Link: http://lkml.kernel.org/r/151844393004.210639.4672319312617954272.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>