udf: Drop pointless check from udf_sync_fs()

The check if (bh) in udf_sync_fs() is pointless as we cannot have
sbi->s_lvid_dirty and !sbi->s_lvid_bh (as already asserted by
udf_updated_lvid()). So just drop the pointless check.

Reviewed-by: Steven J. Magnani <steve@digidescorp.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jan Kara <jack@suse.cz>