btrfs: use smaller type for btrfs_path lowest_level

The level is 0..7, we can use smaller type. The size of btrfs_path is now
136 bytes from 144, which is +2 objects that fit into a 4k slab.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -598,7 +598,7 @@ struct btrfs_path {
 	int locks[BTRFS_MAX_LEVEL];
 	u8 reada;
 	/* keep some upper locks as we walk down */
-	int lowest_level;
+	u8 lowest_level;
 
 	/*
 	 * set by btrfs_split_item, tells search_slot to keep all locks