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

include/os/freebsd/spl/sys/mod_os.h

@@ -72,6 +72,9 @@
 #define	param_set_deadman_ziotime_args(var) \
     CTLTYPE_ULONG, NULL, 0, param_set_deadman_ziotime, "LU"
 
+#define	param_set_multihost_interval_args(var) \
+    CTLTYPE_ULONG, &var, 0, param_set_multihost_interval, "LU"
+
 #define	param_set_slop_shift_args(var) \
     CTLTYPE_INT, &var, 0, param_set_slop_shift, "I"