pf: postpone clearing of struct pf_pdesc

Postpone zeroing out pd until after the PFI_IFLAG_SKIP/M_SKIP_FIREWALL
checks. We don't need it until then, and it saves us a few CPU cycles in
some cases.
This isn't expected to make a measurable performance change though.

Reviewed by: mjg, glebius
Pointed out by: markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33815