raid setup: btrfs: do not allow legacy BIOS boot on 4Kn disks

raid setup: btrfs: do not allow legacy BIOS boot on 4Kn disks

This is already checked for LVM and ZFS setups, but not for Btrfs. Add
it there too, as it doesn't work anyway.

Tested by creating a block device with 4K sectorsize using
the following QEMU args:
  -device virtio-blk,drive=testdrive4k,logical_block_size=4096,physical_block_size=4096
  -drive file=/path/to/4k-testdisk.img,if=none,id=testdrive4k

The 4k-testdisk.img was created with:
  qemu-img create -f qcow2 /path/to/4k-testdisk.img 16G

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>