bridge: simplify ip_mc_check_igmp() and ipv6_mc_check_mld() internals

With this patch the internal use of the skb_trimmed is reduced to
the ICMPv6/IGMP checksum verification. And for the length checks
the newly introduced helper functions are used instead of calculating
and checking with skb->len directly.

These changes should hopefully make it easier to verify that length
checks are performed properly.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>