Btrfs-progs: change the way mkfs picks raid profiles

Btrfs-progs: change the way mkfs picks raid profiles

Currently mkfs in response to

  mkfs.btrfs -d raid10 dev1 dev2

instead of telling "you can't do that" creates a SINGLE on two devices,
and only rebalance can transform it to raid0.  Generally, it never warns
users about decisions it makes and it's not at all obvious which profile
it picks when.

Fix this by checking the number of effective devices and reporting back
if the specified profile is impossible to create.  Do not create FS in
case invalid profile was given.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>