fscrypt: use sha256() instead of open coding

Now that there's a library function that calculates the SHA-256 digest
of a buffer in one step, use it instead of sha256_init() +
sha256_update() + sha256_final().

Link: https://lore.kernel.org/r/20200917045341.324996-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>