Use in_cksum_skip() to calculate in4_cksum instead of doing handrolled magic (that does not compile with !gcc). Moreover we get the benefit for all archs that have a hand optimized in_cksum_skip().
Use in_cksum_skip() to calculate in4_cksum instead of doing handrolled magic
(that does not compile with !gcc). Moreover we get the benefit for all archs
that have a hand optimized in_cksum_skip().

Submitted by:	yongari
Tested by:	me (i386, extensivly), pf4freebsd ML (various)

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