MFC: r336610 (partial), r339207, r339267

MFC: r336610 (partial), r339207, r339267

- Or in the DMA coalescing Rx threshold so the other bits set in E1000_DMACR
  remain intact as intended in igb_init_dmac(). [1]

- Fix igb corrupting checksums with BPF and VLAN
  In stable/11, this merely syncs the code with head as the problem was
  introduced with r311849 in the latter and then fixed by r339207 with
  a different approach than the code used pre-r311849 and in stable/11.

- Use mbuf defines to construct csum offload masks rather than literals

Reported by:	Coverity
CID:		1304929 [1]