ktrace: log genio events on failed write

Visibility into the contents of the buffer when a write(2) has failed
can be immensely useful in debugging IPC issues -- pushing this to
discuss the idea, or maybe an alternative where we can set a flag like
KTRFAC_ERRIO to enable it.

When a genio event is potentially raised after an error, currently we'll
just free the uio and return. However, such data can be useful when
debugging communication between processes to, e.g., understand what the
remote side should have grabbed before closing a pipe. Tap out the
entire buffer on failure rather than simply discarding it.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D43799