FROMLIST: fscrypt: invoke crypto API for ESSIV handling

Instead of open-coding the calculations for ESSIV handling, use an ESSIV
skcipher which does all of this under the hood.  ESSIV was added to the
crypto API in v5.4.

This is based on a patch from Ard Biesheuvel, but reworked to apply
after all the fscrypt changes that went into v5.4.

Tested with 'kvm-xfstests -c ext4,f2fs -g encrypt', including the
ciphertext verification tests for v1 and v2 encryption policies.

Change-Id: I1b09160c0135a2d8b16db7b4139d22b903ec6a8e
Originally-from: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Link: https://patchwork.kernel.org/patch/11182383/