A number of netfront fixes and stability improvements:
A number of netfront fixes and stability improvements:

 - Re-enable TSO.  This was broken previously due to CSUM_TSO clearing the
   CSUM_TCP flag, so our checksum flags were incorrectly set going to the
   netback driver.  That was fixed in r206844 in tcp_output.c, so we can
   turn TSO back on here.

 - Fix the way transmit slots are calculated, so that we can't overfill
   the ring.

 - Avoid sending packets with more fragments/segments than netback can
   handle.  The Linux netback code can only handle packets of
   MAX_SKB_FRAGS, which turns out to be 18 on machines with 4K pages.  We
   can easily generate packets with 32 or so fragments with TSO turned on.
   Right now the solution is just to drop the packets (since netback
   doesn't seem to handle it gracefully), but we should come up with a way
   to allow a driver to tell the TCP stack the maximum number of fragments
   it can handle in a single packet.

 - Fix the way the consumer is tracked in the receive path.  It could get
   out of sync fairly easily.

 - Use standard Xen ring macros to make it clearer how netfront is using
   the rings.

 - Get rid of Linux-ish negative errno return values.

 - Added more documentation to the driver.

 - Refactored code to make it easier to read.

 - Some other minor fixes.

Reviewed by:	gibbs

Reviewed by:	gibbs
Sponsored by:	Spectra Logic
MFC after:	7 days

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