Clean up the kqueue use of the uma KPI.

Explain why it is fine to not check for M_NOWAIT failures in
kqueue_register(). Remove unneeded check for NULL result from
waitable allocation in kqueue_scan(). uma_free(9) handles NULL
argument correctly, remove checks for NULL. Remove useless cast and
adjust style in knote_alloc().

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks