fs: cifs: switch to RC4 library interface

The CIFS code uses the sync skcipher API to invoke the ecb(arc4) skcipher,
of which only a single generic C code implementation exists. This means
that going through all the trouble of using scatterlists etc buys us
very little, and we're better off just invoking the arc4 library directly.

This also reverts commit 5f4b5569 ("CIFS: Fix BUG() in calc_seckey()"),
since it is no longer necessary to allocate sec_key on the heap.

Cc: linux-cifs@vger.kernel.org
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>