MFC r337812,r337814,r337820,r341068:

r337812 also fixed a bug in the netmap kevent code. The inclusion of
that fix was an oversight that I didn't notice until this
MFC. Reference the code review and PR here in the MFC for
completeness.

PR:  206053
Differential Revision:    https://reviews.freebsd.org/D16531
Sponsored by:	Dell EMC Isilon