igmp: convert igmpstat to use PCPU counters

Currently there is no locking done to protect this structure. It is
likely okay due to the low-volume nature of IGMP, but allows for
the possibility of underflow. This appears to be one of the only
holdouts of the conversion to counter(9) which was done for most
protocol stat structures around 2013.

This also updates the visibility of this stats structure so that it can
be consumed from elsewhere in the kernel, consistent with the vast
majority of VNET_PCPUSTAT structures.

Reviewed by: kp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27023