crypto: skcipher - Unmap pages after an external error

skcipher_walk_done may be called with an error by internal or
external callers.  For those internal callers we shouldn't unmap
pages but for external callers we must unmap any pages that are
in use.

This patch distinguishes between the two cases by checking whether
walk->nbytes is zero or not.  For internal callers, we now set
walk->nbytes to zero prior to the call.  For external callers,
walk->nbytes has always been non-zero (as zero is used to indicate
the termination of a walk).

Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: 5cde0af2 ("[CRYPTO] cipher: Added block cipher type")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>