pf: simplify pf_check_proto_cksum()

Simplify and shorten the way ICMP checksums are verified in
pf_check_proto_cksum() by letting it use the same in4_cksum() call that
is used for TCP and UDP checksums.

ok henning@ naddy@

Obtained from: OpenBSD, lteo <lteo@openbsd.org>, 3c23016fb7
Sponsored by: Rubicon Communications, LLC ("Netgate")