btrfs: remove unnecessary curly braces in btrfs_get_acl

It's only coding style fix not functinal change.  When if/else has only
one statement then the braces are not needed.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>