Fix if_vmx receive checksum offload bug and harden against the device skipping receive descriptors

This fixes a bug where the checksum offload status of received packets
was being taken from the first descriptor instead of the last, which
affected LRO packets.

The driver has been hardened against the device skipping receive
descriptors, although it is not believed that this can occur given the
way this implementation configures the receive rings.

Additionally, for packets received with the error indicator set, the
driver now forces the length of all fragments in that packet to zero
prior to passing it to iflib. Such packets should wind up being
discarded at some point in the stack anyway, but this removes any
questions by killing them in the driver.

Counters have been added (and exposed via sysctls) for skipped receive
descriptors, zero-length packets received, and packets received with
the error indicator set so that these conditions can be easily
observed in the field.

PR: 243126, 243392, 240628
Reported by: avg, alexandr.oleynikov@gmail.com, Harald Schmalzbauer
Reviewed by: gallatin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23949