Refactor deadman set failmode to be cross platform

Update zfs_deadman_failmode to use the ZFS_MODULE_PARAM_CALL
wrapper, and split the common and platform specific portions.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9670

module/os/linux/zfs/spa_misc_os.c

@@ -42,6 +42,19 @@
 #include <sys/kstat.h>
 #include "zfs_prop.h"
 
+
+int
+param_set_deadman_failmode(const char *val, zfs_kernel_param_t *kp)
+{
+	int error;
+
+	error = -param_set_deadman_failmode_common(val);
+	if (error == 0)
+		error = param_set_charp(val, kp);
+
+	return (error);
+}
+
 int
 param_set_deadman_ziotime(const char *val, zfs_kernel_param_t *kp)
 {