module param callbacks check for initialized spa

Callbacks provided for module parameters are executed both
after the module is loaded, when a user alters it via sysfs, e.g
	echo bar > /sys/modules/zfs/parameters/foo

as well as when the module is loaded with an argument, e.g.
	modprobe zfs foo=bar

In the latter case, the init functions likely have not run yet,
including spa_init() which initializes the namespace lock so it is safe
to use.

Instead of immediately taking the namespace lock and attemping to
iterate over initialized spa structures, check whether spa_mode_global
is nonzero.  This is set by spa_init() after it has initialized the
namespace lock.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7496 
Closes #7521

module/zfs/mmp.c

@@ -609,7 +609,8 @@ param_set_multihost_interval(const char *val, zfs_kernel_param_t *kp)
 	if (ret < 0)
 		return (ret);
 
-	mmp_signal_all_threads();
+	if (spa_mode_global != 0)
+		mmp_signal_all_threads();
 
 	return (ret);
 }