Fix more leaks detected by ASAN

This commit fixes a bunch of missing free() calls in a10d50f

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: João Carlos Mendes Luís <jonny@jonny.eng.br>
Closes openzfs#10219 
(cherry picked from commit 70e5ad3)

lib/libzfs/libzfs_dataset.c

@@ -931,10 +931,15 @@ libzfs_mnttab_add(libzfs_handle_t *hdl, const char *special,
 		 * Another thread may have already added this entry
 		 * via libzfs_mnttab_update. If so we should skip it.
 		 */
-		if (avl_find(&hdl->libzfs_mnttab_cache, mtn, NULL) != NULL)
+		if (avl_find(&hdl->libzfs_mnttab_cache, mtn, NULL) != NULL) {
+			free(mtn->mtn_mt.mnt_special);
+			free(mtn->mtn_mt.mnt_mountp);
+			free(mtn->mtn_mt.mnt_fstype);
+			free(mtn->mtn_mt.mnt_mntopts);
 			free(mtn);
-		else
+		} else {
 			avl_add(&hdl->libzfs_mnttab_cache, mtn);
+		}
 	}
 	pthread_mutex_unlock(&hdl->libzfs_mnttab_cache_lock);
 }