gss_krb5: Fix the error handling path for crypto_sync_skcipher_setkey

If we fail to call crypto_sync_skcipher_setkey, we should free the
memory allocation for cipher, replace err_return with err_free_cipher
to free the memory of cipher.

Fixes: 4891f2d0 ("gss_krb5: import functionality to derive keys into the kernel")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>