common/encrypt: check that contents encryption is usable

common/encrypt: check that contents encryption is usable

In _require_encryption_policy_support(), when checking whether the
encryption policy is usable, try creating a nonempty file rather
than an empty one.  This ensures that both the contents and
filenames encryption modes are available, rather than just the
filenames mode.

On f2fs this makes generic/549 be correctly skipped, rather than
failed, when run on a kernel built from the latest fscrypt.git tree
with CONFIG_CRYPTO_SHA256=n.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>