carp: Fix checking IPv4 multicast address

An IPv4 address stored in struct in_addr is in network byte order but
IN_MULTICAST wants host order.

PR: 284872
Reported by: Steven Perreau
Reported by: Brett Merrick <brett.merrick@itcollective.nz>
Reviewed by: Franco Fichtner <franco@opnsense.org>, ae, kp, glebius
Tested by: Steven Perreau
Fixes: 137818006de5 carp: support unicast
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D49053