btrfs: fix devs_max constraints for raid1c3 and raid1c4

The value 0 for devs_max means to spread the allocated chunks over all
available devices, eg. stripe for RAID0 or RAID5. This got mistakenly
copied to the RAID1C3/4 profiles. The intention is to have exactly 3 and
4 copies respectively.

Fixes: 47e6f742 ("btrfs: add support for 3-copy replication (raid1c3)")
Fixes: 8d6fac00

 ("btrfs: add support for 4-copy replication (raid1c4)")
Signed-off-by: David Sterba <dsterba@suse.com>