ip_mroute: Make privilege checking more consistent

• The v6 socket option and ioctl handlers had no privilege checks at all. The socket options, I believe, can only be reached via a raw socket, but a jailed root user with a raw socket shouldn't be able to configure multicast routing in a non-VNET jail. The ioctls can only be used to fetch stats.
• Delete a bogus comment in X_mrt_ioctl(), one can issue multicast routing ioctls against any socket. Note that the call path is soo_ioctl()->rtioctl_fib()->mrt_ioctl().

I think all of the mroute privilege checks should be done within the
ip(6)_mroute code, but let's first make the v4 and v6 modules
consistent.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54982