Factor Linux specific code out of spa_misc.c

Move these Linux module parameter get/set helpers in to
platform specific code.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9457

include/os/linux/kernel/linux/mod_compat.h

@@ -53,6 +53,7 @@ enum scope_prefix_types {
 	zfs_condense,
 	zfs_dbuf,
 	zfs_dbuf_cache,
+	zfs_deadman,
 	zfs_l2arc,
 	zfs_livelist,
 	zfs_livelist_condense,
@@ -71,6 +72,7 @@ enum scope_prefix_types {
 	zfs_vdev_mirror,
 	zfs_zio,
 	zfs_zil,
+	spa
 };
 
 /*