btrfs: add back warning for mount option commit values exceeding 300

The Btrfs documentation states that if the commit value is greater than
300 a warning should be issued. The warning was accidentally lost in the
new mount API update.

Fixes: 6941823c ("btrfs: remove old mount API code")
CC: stable@vger.kernel.org # 6.12+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Kyoji Ogasawara <sawara04.o@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>