netinet: Implement in_cksum_skip() using m_apply()

This allows it to work with unmapped mbufs. In particular,
in_cksum_skip() calls no longer need to be preceded by calls to
mb_unmapped_to_ext() to avoid a page fault.

PR: 259645
Reviewed by: gallatin, glebius, jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 0d9c3423f59bb305301f5a5bc7c8f5daf7b7aa52)