Fix invalid pointer access in trace_dbuf.h

In dnode_destroy, dn_objset is invalidated. However, it will later call
into dbuf_destroy, in which DTRACE_SET_STATE will try to access spa_name
via dn_objset causing illegal pointer access.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes openzfs#15333

include/os/linux/zfs/sys/trace_dbuf.h

@@ -60,8 +60,12 @@
 
 #define	DBUF_TP_FAST_ASSIGN						\
 	if (db != NULL) {						\
-		__assign_str(os_spa,					\
-		spa_name(DB_DNODE(db)->dn_objset->os_spa));		\
+		if (POINTER_IS_VALID(DB_DNODE(db)->dn_objset)) {	\
+			__assign_str(os_spa,				\
+			spa_name(DB_DNODE(db)->dn_objset->os_spa));	\
+		} else {						\
+			__assign_str(os_spa, "NULL");			\
+		}							\
 									\
 		__entry->ds_object = db->db_objset->os_dsl_dataset ?	\
 		db->db_objset->os_dsl_dataset->ds_object : 0;		\