Refactor to use IGMP_V3_REPORT_MINLEN and IGMP_GRPREC_HDRLEN.

The optional .ir_groups[] struct member in struct igmp_report does
not exist on FreeBSD, it's been commented out.  Replace that, and
sizeof(struct igmp_grouprec) with IGMP_V3_REPORT_MINLEN and
IGMP_GRPREC_HDRLEN, respectively for compatibility.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>