Most Ethernet drivers that potentially can run a pfil(9) hook with
PFIL_MEMPTR flag are intentionally providing a memory address that
isn't aligned to pointer alignment. This is done to align an IPv4
or IPv6 header that is expected to follow Ethernet header.

When we return PFIL_REALLOCED we store a pointer to allocated mbuf
at this address. With this change the KPI changes to store the pointer
at aligned address, which usually yields in +2 bytes.

Provide two inlines:

pfil_packet_align() to get aligned pfil_packet_t for a misaligned one
pfil_mem2mbuf() to read out mbuf pointer from misaligned pfil_packet_t

Provide function pfil_realloc(), not used yet, that would convert a
memory pfil_packet_t to an mbuf one.

Reported by: hps
Reviewed by: hps, gallatin