pimd: IGMPv2 leave msg may be longer than 8 octets

pimd: IGMPv2 leave msg may be longer than 8 octets

Fix the code as per RFC 2236 section 2.5:
   Note that IGMP messages may be longer than 8 octets, especially
   future backwards-compatible versions of IGMP.  As long as the Type is
   one that is recognized, an IGMPv2 implementation MUST ignore anything
   past the first 8 octets while processing the packet.  However, the
   IGMP checksum is always computed over the whole IP payload, not just
   over the first 8 octets.

Fixes: #10331
Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>