mbuf: Save 16 bytes from pkthdr on x86_64
- 'wlan_seqno' is not necessary, reuse the 'ether_vlantag'
- Not all parts of 'pkthdr_br' is useful; save the ethernet source
address should be enough.
- Move 'len' after 'header', on x86_64:
o Remove the implicit 4 bytes padding
o 'len' is still in the same cacheline as before this commit
(m_hdr is 160 bytes as of this commit)
o Make the size of the fields after 'header' but before the 'pf'
8 bytes aligned, so there will be on implicit padding before 'pf'