ZTS: Improve block_device_wait on FreeBSD

FreeBSD doesn't have an equivalent to udevadm settle, so we have been
resorting to a three second sleep to wait for device changes to take
effect.  This is far from ideal.

We are mainly waiting for volmode=geom zvols to appear in /dev, so as
a hack, reading the geom config will have the desired effect of
quiescing the geom state.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10768

tests/zfs-tests/include/blkdev.shlib

@@ -81,7 +81,9 @@ function block_device_wait
 		    log_note udevadm settle time too long: $elapsed
 	elif is_freebsd; then
 		if [[ ${#@} -eq 0 ]]; then
-			sleep 3
+			# Do something that has to go through the geom event
+			# queue to complete.
+			sysctl kern.geom.conftxt >/dev/null
 			return
 		fi
 	fi