common/encrypt: Create an encrypted equivalent of _scratch_mkfs_sized

common/encrypt: Create an encrypted equivalent of _scratch_mkfs_sized

Test case generic/399 hardcodes "-O encrypt" in MKFS_OPTIONS when
calling _scratch_mkfs_sized, which only works with the mkfs of
certain filesystems. Create a new helper,
_scratch_mkfs_sized_encrypted, for handling the differences between
the mkfs tools of different filesystems. It also allows those
filesystems whose mkfs doesn't accept "-O encrypt" to skip the test
gracefully until proper support is added for them in the helper.

ubifs is not supported in the new helper despite supporting
encryption, as _scratch_mkfs_sized has no ubifs support and adding
that should be done in a separate patch.

Signed-off-by: Ari Sundholm <ari@tuxera.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>