Fixup packet fragment unrolling procedure

Fixup packet fragment unrolling procedure

When bus_dmamap_load fails to load one of the buffers in the mbuf
chain, we need to revert all changes to transmit descriptors.  The
code to do that was prototyped but not tested.  However due to how
the Tx ring is set up in xnf(4) and generic lack of proper fragment
support in the Netfront design we're always limited to having 256
entries for distinct shared memory pages.  The mbuf chain is
traversed and attempt is made to load every data chunk into a 4k
sized DMA map segment which makes it impossible to reference a
buffer composed of multiple pages.  Current implementation lacks
support for this preventing reliable transmission of frames larger
than 4k.

Bug reported by Kirill Miazine <km at krot ! org>, thanks!