btrfs: output the reason for open_ctree() failure

commit d0f0381 upstream.

There is a recent ML report that mounting a large fs backed by hardware
RAID56 controller (with one device missing) took too much time, and
systemd seems to kill the mount attempt.

In that case, the only error message is:

  BTRFS error (device sdj): open_ctree failed

There is no reason on why the failure happened, making it very hard to
understand the reason.

At least output the error number (in the particular case it should be
-EINTR) to provide some clue.

Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Reported-by: Christoph Anton Mitterer <[email protected]>
Cc: [email protected]
Reviewed-by: Filipe Manana <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: adam900710

fs/btrfs/super.c

@@ -971,7 +971,7 @@ static int btrfs_fill_super(struct super_block *sb,
 
 	err = open_ctree(sb, fs_devices);
 	if (err) {
-		btrfs_err(fs_info, "open_ctree failed");
+		btrfs_err(fs_info, "open_ctree failed: %d", err);
 		return err;
 	}