Btrfs: fix em leak in find_first_block_group

We need to call free_extent_map() on the em we look up.
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -9887,6 +9887,7 @@ static int find_first_block_group(struct btrfs_root *root,
 			} else {
 				ret = 0;
 			}
+			free_extent_map(em);
 			goto out;
 		}
 		path->slots[0]++;