Btrfs end_bio_extent_readpage should look for locked bits

A recent commit caches the extent state in end_bio_extent_readpage,
but the search it does should look for locked extents.  This
fixes things to make it more effective.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -1761,7 +1761,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 			prefetchw(&bvec->bv_page->flags);
 
 		spin_lock(&tree->lock);
-		state = find_first_extent_bit_state(tree, start, 0);
+		state = find_first_extent_bit_state(tree, start, EXTENT_LOCKED);
 		if (state && state->start == start) {
 			/*
 			 * take a reference on the state, unlock will drop