find_lock_page does not need to recheck ->index because if the page is in the
right mapping then the index must be the same.  Also, tree_lock does not need
to be retaken after the page is locked in order to test that ->mapping has not
changed, because holding the page lock pins its mapping.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>