Correct suspend/resume support in the Netfront driver.
Correct suspend/resume support in the Netfront driver.

Sponsored by: BQ Internet

sys/dev/xen/netfront/netfront.c:
	o Implement netfront_suspend(), a specialized suspend
	  handler for the netfront driver.  This routine simply
	  disables the carrier so the driver is idle during
	  system suspend processing.
	o Fix a leak when re-initializing LRO during a link reset.
	o In netif_release_tx_bufs(), when cleaning up the grant
	  references for our TX ring, use gnttab_end_foreign_access_ref
	  instead of attempting to grant the page again.
	o In netif_release_tx_bufs(), we do not track mbufs associated
	  with mbuf chains, but instead just free each mbuf directly.
	  Use m_free(), not m_freem(), to avoid double frees of mbufs.
	o Refactor some code to enhance clarity.

Approved by:	re
MFC after:	1 week

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