[CIFS] Fix plaintext authentication

The last eight bytes of the password field were not cleared when doing lanman plaintext password authentication. This patch fixes that.

I tested it with Samba by setting password
encryption to no in the server's smb.conf.  Other servers also can be
configured to force plaintext authentication.    Note that plaintexti
authentication requires setting /proc/fs/cifs/SecurityFlags to 0x30030
on the client (enabling both LANMAN and also plaintext password support).
Also note that LANMAN support (and thus plaintext password support) requires
CONFIG_CIFS_WEAK_PW_HASH to be enabled in menuconfig.

CC: Jeff Layton <jlayton@redhat.com>
CC: Stable Kernel <stable@vger.kernel.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>