btrfs/012: check if ext4 is available

btrfs/012: check if ext4 is available

btrfs/012 is requiring ext4 support to test the conversion, but the test
case is only checking if mkfs.ext4 is available, not if the filesystem
driver is actually available on the test host.

Check if the driver is available as well, before trying to run the test.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>