generic: move test case btrfs/261 into the generic group

generic: move test case btrfs/261 into the generic group

The test case btrfs/261, part from its comments, doesn't really exercise
any behaviour that is btrfs specific, so, as Dave Chinner pointed out, it
can be moved into the generic group.

This change moves that test into the generic group and slightly adjust the
comments to make it clear which parts are btrfs specific.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>