Handle multiple calls to rxq_eof for single packet completion

This requires the VMware vmxnet3 device to flip the start of packet
descriptor's generation before the rest of the packet's descriptors
have been loaded into the Rx ring. I've never observed this behavior,
and it seems to make the most sense not to do it this way. But it is
not a lot of work for the driver to handle this situation just in case.

MFC after:	1 week
Increment the pending packets more aggressively for TSO

Assume the number of description used is reasonable value to
increment this otherwise opaque field by.

While here, reduce a minor difference between the legacy and
multiqueue transmit paths.

MFC after:	1 week