Btrfs: fix em leak in find_first_block_group

BugLink: https://bugs.launchpad.net/bugs/1818237

commit 187ee58c upstream.

We need to call free_extent_map() on the em we look up.
Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/btrfs/extent-tree.c

@@ -9518,6 +9518,7 @@ static int find_first_block_group(struct btrfs_root *root,
 			} else {
 				ret = 0;
 			}
+			free_extent_map(em);
 			goto out;
 		}
 		path->slots[0]++;