digsig: Fix memory leakage in digsig_verify_rsa()

digsig_verify_rsa() does not free kmalloc'ed buffer returned by
mpi_get_buffer().

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>

lib/digsig.c

@@ -162,6 +162,8 @@ static int digsig_verify_rsa(struct key *key,
 	memset(out1, 0, head);
 	memcpy(out1 + head, p, l);
 
+	kfree(p);
+
 	err = pkcs_1_v1_5_decode_emsa(out1, len, mblen, out2, &len);
 	if (err)
 		goto err;