dm zoned: Fix metadata zone size check

When dm zoned has multiple devices, metadata is on the cache device, not
in random zones of the zoned devices. Then the number of metadata zones
shall be checked with the number of cache zones, not random zones.

Fixes: 34f5affd ("dm zoned: separate random and cache zones")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>