pf: copy out rather than m_pullup() in pf_test_eth_rule()

Don't change the mbuf chain layout. We've encountered alignment issues
in the tcp syncookie code on armv7, which are triggered by the
m_pullup() here.

Reviewed by: mjg
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35551