Add zfs_multihost_interval tunable handler for FreeBSD

This tunable required a handler to be implemented for
ZFS_MODULE_PARAM_CALL.

Add the handler so the tunable can be declared in common code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10490

module/zfs/mmp.c

@@ -715,6 +715,12 @@ mmp_signal_all_threads(void)
 	mutex_exit(&spa_namespace_lock);
 }
 
+/* BEGIN CSTYLED */
+ZFS_MODULE_PARAM_CALL(zfs_multihost, zfs_multihost_, interval,
+	param_set_multihost_interval, param_get_ulong, ZMOD_RW,
+	"Milliseconds between mmp writes to each leaf");
+/* END CSTYLED */
+
 ZFS_MODULE_PARAM(zfs_multihost, zfs_multihost_, fail_intervals, UINT, ZMOD_RW,
 	"Max allowed period without a successful mmp write");