eCryptfs: Fix null pointer dereference on kzalloc error path

The conversion to skcipher and shash added a couple of null pointer
dereference bugs on the kzalloc failure path.  This patch fixes them.

Fixes: 3095e8e3 ("eCryptfs: Use skcipher and shash")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>