mm: add lock release annotation on do_wp_page()

The do_wp_page() releases @ptl but was missing proper annotation.  Add it.
 This removes following warnings from sparse:

 mm/memory.c:2337:9: warning: context imbalance in 'do_wp_page' - unexpected unlock
 mm/memory.c:3142:19: warning: context imbalance in 'handle_mm_fault' - different lock contexts for basic block

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>