Revert "btrfs: reject unknown mount options early"

This reverts commit 5f521494.

The patch breaks mounts with security mount options like

  $ mount -o context=system_u:object_r:root_t:s0 /dev/sdX /mn
  mount: /mnt: wrong fs type, bad option, bad superblock on /dev/sdX, missing codepage or helper program, ...

We cannot reject all unknown options in btrfs_parse_subvol_options() as
intended, the security options can be present at this point and it's not
possible to enumerate them in a future proof way. This means unknown
mount options are silently accepted like before when the filesystem is
mounted with either -o subvol=/path or as followup mounts of the same
device.

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -954,10 +954,6 @@ static int btrfs_parse_subvol_options(const char *options, char **subvol_name,
 
 			*subvol_objectid = subvolid;
 			break;
-		case Opt_err:
-			btrfs_err(NULL, "unrecognized mount option '%s'", p);
-			error = -EINVAL;
-			goto out;
 		default:
 			break;
 		}