Remove dead code since r230426, fix a comment, and tidy up.

Reported by:	jhb
MFC after:	3 days
Remove ifa_ref()/ifa_free(), which are atomic(9), from ip_output().

The ifaddr is already referenced by the rtentry, and we are holding
reference on the rtentry throughout the function execution.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Properly save and restore CR0.

MFC after:	3 days
Allow comments at end of line.

Reviewed by: marcel
Revert accidentally committed changes in 262748.