Update to use reworked SPL debugging

This change updates the ZFS code to use the slightly reworked
SPL debug infrastructure.  It also explicitly sets all ZFS
dprintf debugging to use the SS_USER1 subsystem for logging
in the SPL debug log.

module/zfs/include/sys/zfs_debug.h

@@ -55,8 +55,16 @@ extern int zfs_flags;
 
 #ifdef ZFS_DEBUG
 #if defined(_KERNEL) && defined(HAVE_SPL)
-#include <sys/debug.h>
-#define dprintf(...) CDEBUG_LIMIT(D_DPRINTF, __VA_ARGS__)
+/*
+ * Log ZFS debug messages as the spl SS_USER1 subsystem.
+ */
+#include <spl-debug.h>
+
+#ifdef SS_DEBUG_SUBSYS
+#undef SS_DEBUG_SUBSYS
+#endif
+#define SS_DEBUG_SUBSYS SS_USER1
+#define dprintf(...) SDEBUG_LIMIT(SD_DPRINTF, __VA_ARGS__)
 #else
 extern void __dprintf(const char *file, const char *func,
     int line, const char *fmt, ...);