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'
mtag: u_intXX_t -> uintXX_t; no functional changes
mbuf: Add comment about the remaining implicit padding on x86_64