tpm: fix a kernel memory leak in tpm-sysfs.c

While cleaning up sysfs callback that prints EK we discovered a kernel
memory leak. This commit fixes the issue by zeroing the buffer used for
TPM command/response.

The leak happen when we use either tpm_vtpm_proxy, tpm_ibmvtpm or
xen-tpmfront.

Cc: stable@vger.kernel.org
Fixes: 08837438 ("TPM: sysfs functions consolidation")
Reported-by: NJason Gunthorpe <jgunthorpe@obsidianresearch.com>
Tested-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: NJarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>

drivers/char/tpm/tpm-sysfs.c

@@ -36,9 +36,10 @@ static ssize_t pubek_show(struct device *dev, struct device_attribute *attr,
 	ssize_t err;
 	int i, rc;
 	char *str = buf;
-
 	struct tpm_chip *chip = to_tpm_chip(dev);
 
+	memset(&tpm_cmd, 0, sizeof(tpm_cmd));
+
 	tpm_cmd.header.in = tpm_readpubek_header;
 	err = tpm_transmit_cmd(chip, NULL, &tpm_cmd, READ_PUBEK_RESULT_SIZE,
 			       READ_PUBEK_RESULT_MIN_BODY_SIZE, 0,