libzfs: Fix mounting datasets under thread limit pressure

During parallel zpool import, /sbin/zpool will create a separate thread
pool for each pool, used to mount that pool's datasets.  If the total
thread count exceed's the system's limit on threads per process, then
tpool_dispatch may fail.  If it does, directly execute the mount
operation instead.

Sponsored by: Axcient
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Alan Somers <asomers@FreeBSD.org>
Closes #16178
Fixes #16172

lib/libzfs/libzfs_mount.c

@@ -1098,7 +1098,10 @@ zfs_dispatch_mount(libzfs_handle_t *hdl, zfs_handle_t **handles,
 	mnt_param->mnt_func = func;
 	mnt_param->mnt_data = data;
 
-	(void) tpool_dispatch(tp, zfs_mount_task, (void*)mnt_param);
+	if (tpool_dispatch(tp, zfs_mount_task, (void*)mnt_param)) {
+		/* Could not dispatch to thread pool; execute directly */
+		zfs_mount_task((void*)mnt_param);
+	}
 }
 
 /*