[CIFS] fix error message about packet signing

When packet signing is disabled and the server requires it, cifs prints
an error message. The current message refers to a file in /proc that no
longer exists. Fix it to refer to the correct file.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>