common/rc: avoid mixed mode for zoned btrfs

When the helper function _scratch_mkfs_sized is called with small
filesystem size and FSTYP=btrfs, it calls mkfs.btrfs with --mixed option
to enable mixed mode. However, mkfs.btrfs with --mixed option fails for
zoned block devices since btrfs does not support mixed mode together
with zoned mode. To avoid this failure, do not set --mixed option when
the scratch device is a zoned block device.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>