ips - Issue IPS_CACHE_FLUSH_CMD to the controller on BUF_CMD_FLUSH

ips - Issue IPS_CACHE_FLUSH_CMD to the controller on BUF_CMD_FLUSH

Previously, BUF_CMD_FLUSH ended up as a zero-byte write command, which
always fails, flooding the console with `iobuf error 5'.  Filesystems
other than HAMMER almost never issues this command, so we've never
seen the error message in pre-HAMMER days.  This commit adds a new path
for BUF_CMD_FLUSH and issue IPS_CACHE_FLUSH_CMD for it.

Also mention the tunable/sysctl knob debug.ips.ignore_flush_cmd in ips(4)
man page in case the new behavior confuses your controller; when set, the
driver just discards BUF_CMD_FLUSH.