TPM: Zero buffer whole after copying to userspace

commit 3ab1aff89477dafb1aaeafe8c8669114a02b7226 upstream.

Commit 3321c07ae5068568cd61ac9f4ba749006a7185c9 correctly clears the TPM
buffer if the user specified read length is >= the TPM buffer length. However,
if the user specified read length is < the TPM buffer length, then part of the
TPM buffer is left uncleared.

Reported-by: Seth Forshee <seth.forshee@canonical.com>
Cc: Debora Velarde <debora@linux.vnet.ibm.com>
Cc: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Cc: Marcel Selhorst <m.selhorst@sirrix.com>
Cc: tpmdd-devel@lists.sourceforge.net
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>