pf: Introduce M_PF type for pf(4) related memory allocations.

Currently used M_TEMP and M_IFADDR types are unreasonable for that purpose.
This dedicated statistics simplify the future pf(4) unlocking work by decreasing
search area of possible memory leaks.

ok bluhm sashan

FreeBSD note: The unlocking work has already been done in FreeBSD, but it's
still useful to have all pf malloc() allocations be accounted to pf, not the
generic 'temp' bucket.

Obtained from: OpenBSD, mvs <mvs@openbsd.org>, 062cda8b8d
Sponsored by: Rubicon Communications, LLC ("Netgate")