if_stf: KASAN fix

In in_stf_input() we grabbed a pointer to the IPv4 header and later did
an m_pullup() before we look at the IPv6 header. However, m_pullup()
could rearrange the mbuf chain and potentially invalidate the pointer to
the IPv4 header.

Avoid this issue by copying the IP header rather than getting a pointer
to it.

Reported by: markj, Jenkins (KASAN job)
Reviewed by: markj
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33192