btrfs send <ro-snap-0> -p <ro-snap-1>

Hi guys,

It seems that btrfs v4.12.1 allows:

	(1) btrfs send -p <ro-snap-1> <ro-snap-0>

but disallows

	(2) btrfs send <ro-snap-0> -p <ro-snap-1>

Code-wise it assumes that <ro-snap-1> is always found at optind == 1. I was about to patch this but I'm not sure which way we'd like to go with this:
Actually only allow (1) and block (2) properly or allow both. In any case, this seems to me like a pretty serious regression and we'd like to get this settled soon. :)

Thanks!
Christian