Allow disabling of unmapped I/O on FreeBSD

We have a tunable which permits one to disable the use of unmapped I/O
for the buffer cache.  Respect it in ZFS as well.  This is useful for
KMSAN, which cannot easily maintain shadow state for unmapped pages.

No functional change intended, as unmapped I/O is permitted by default
and there's no real reason to disable it in practice except for
debugging.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes openzfs#12446

module/os/freebsd/zfs/vdev_geom.c

@@ -1100,6 +1100,10 @@ vdev_geom_check_unmapped(zio_t *zio, struct g_consumer *cp)
 {
 	struct vdev_geom_check_unmapped_cb_state s;
 
+	/* If unmapped I/O is administratively disabled, respect that. */
+	if (!unmapped_buf_allowed)
+		return (0);
+
 	/* If the buffer is already linear, then nothing to do here. */
 	if (abd_is_linear(zio->io_abd))
 		return (0);