fix vmxnet3 crash when LRO is enabled

The crash can occur when all of the following conditions are true:

• a packet consists of multiple segements (requires LRO enabled)
• there has been a failure to allocate an mbuf for the packet and the packet has to be dropped
• a host (vmware) still owned at least one segment of the packet, so the driver had to wait for another interrupt to proceed to discarding the remaning segment(s)

Reviewed by: rstone
MFC after: 2 weeks
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D10874