mmp: add spa_load_name() for tryimport

Tryimport adds a unique prefix to the pool name to avoid name
collisions.  This makes it awkward to log user-friendly info
during a tryimport.  Add a spa_load_name() function which can
be used to report the unmodified pool name.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>

include/sys/spa_impl.h

@@ -222,6 +222,7 @@ struct spa {
 	 * Fields protected by spa_namespace_lock.
 	 */
 	char		spa_name[ZFS_MAX_DATASET_NAME_LEN];	/* pool name */
+	char		*spa_load_name;		/* unmodified pool name */
 	char		*spa_comment;		/* comment */
 	avl_node_t	spa_avl;		/* node in spa_namespace_avl */
 	nvlist_t	*spa_config;		/* last synced config */