spa_open_common: fix argument to zvol_create_minors

Prior to r248571 spa_open was always called with a bare pool name,
but now it is called with a dataset name instead (spa_lookup handles
that).
So, when a ZFS root is mounted spa_open is called with a name of a root
dataset, which can very well be different from the pool name.
But zvol_create_minors should be called with the pool name, because it
performs a recursive traversal of all datasets under the name to find
all those that are volumes.

MFC after:	7 days

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c

@@ -2885,7 +2885,7 @@ spa_open_common(const char *pool, spa_t **spapp, void *tag, nvlist_t *nvpolicy,
 #ifdef __FreeBSD__
 #ifdef _KERNEL
 		if (firstopen)
-			zvol_create_minors(pool);
+			zvol_create_minors(spa->spa_name);
 #endif
 #endif
 	}