pf: Save the protocol number in the pf_fragment

When we try to look up a pf_fragment with pf_find_fragment() we compare (see
pf_frag_compare()) addresses (and family), id but also protocol. We failed to
save the protocol to the pf_fragment in pf_fragcache(), resulting in failing
reassembly.

Differential Revision: https://reviews.freebsd.org/D2772