ubifs: fix wrong use of crypto_shash_descsize()

crypto_shash_descsize() returns the size of the shash_desc context
needed to compute the hash, not the size of the hash itself.

crypto_shash_digestsize() would be correct, or alternatively using
c->hash_len and c->hmac_desc_len which already store the correct values.
But actually it's simpler to just use stack arrays, so do that instead.

Fixes: 49525e5e ("ubifs: Add helper functions for authentication support")
Fixes: da8ef65f ("ubifs: Authenticate replayed journal")
Cc: <stable@vger.kernel.org> # v4.20+
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>