fscrypt: restrict IV_INO_LBLK_* to AES-256-XTS

fscrypt: restrict IV_INO_LBLK_* to AES-256-XTS

IV_INO_LBLK_* exist only because of hardware limitations, and currently
the only known use case for them involves AES-256-XTS.  Therefore, for
now only allow them in combination with AES-256-XTS.  This way we don't
have to worry about them being combined with other encryption modes.

(To be clear, combining IV_INO_LBLK_* with other encryption modes
*should* work just fine.  It's just not being tested, so we can't be
100% sure it works.  So with no known use case, it's best to disallow it
for now, just like we don't allow other weird combinations like
AES-256-XTS contents encryption with Adiantum filenames encryption.)

This can be relaxed later if a use case for other combinations arises.

Fixes: b103fb7653ff ("fscrypt: add support for IV_INO_LBLK_64 policies")
Fixes: e3b1078bedd3 ("fscrypt: add support for IV_INO_LBLK_32 policies")
Link: https://lore.kernel.org/r/20200721181012.39308-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>