• Fixed memory leaks around m_dup() not freeing the original chain on failure. If we return ENOMEM, we are expected to have freed the chain, else the mbuf would be leaked. Also updated iflib_ether_pad() to follow the same structure.

• In iflib_parse_header() o Fixed a bug where the ip/ip6 and th pointers may point into a freed chain after m_pullup. Those pointers must be reset to point into the new chain.
  
  o Eliminate ENXIO returns for non-TCP TSO sends (which would violate the mbuf ownership contract if they could happen). Since they cannot, I made them assertions instead.

• in iflib_ether_pad(), return ENOMEM after freeing mbuf, so that mp_ring knows it is free. An ENOBUFS error will cause the mp_ring path to retain the mbuf and retry

• in iflib_encap(): o Fix a leak when bus_dmamap_load_mbuf_sg() returns ENOMEM
  
  o Fix a use-after-free in the mp_ring path when a driver using ktls frees an mbuf and returns ENOBUFS via iflib_encap()

After this change:

mp_ring: ENOBUFS can be returned only when we run out of descriptors and the mbuf is retained

	 in that case.

simple_tx: iflib_encap() always consumes the mbuf, regardless of the return

Note that iflib_debugnet_transmit(), like simple_tx, expects that iflib_encap() always consumes
mbufs. This will be true after mp_ring is removed, and its such a rare special
case (panic dumps) that I don't think its worth fixing in the meantime.