This patch is for stable/11.
Netmap supports the case where TX rings and RX rings have different size.
Remove unnecessary limitations related to netmap support, making the code simpler.
Also, check that the value of the hw head index written back from the NIC is valid.
It looks like the value read is invalid for some reason... see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230465