btrfs: do not try to unlock the extent for non-subpage metadata reads

Only subpage metadata reads lock the extent.  Don't try to unlock it and
waste cycles in the extent tree lookup for PAGE_SIZE or larger metadata.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4208,8 +4208,10 @@ static void extent_buffer_read_end_io(struct btrfs_bio *bbio)
 		bio_offset += bvec->bv_len;
 	}
 
-	unlock_extent(&bbio->inode->io_tree, eb->start,
-		      eb->start + bio_offset - 1, NULL);
+	if (eb->fs_info->nodesize < PAGE_SIZE) {
+		unlock_extent(&bbio->inode->io_tree, eb->start,
+			      eb->start + bio_offset - 1, NULL);
+	}
 	free_extent_buffer(eb);
 
 	bio_put(&bbio->bio);