cifs: mask off top byte in get_rfc1002_length()

The rfc1002 length actually includes a type byte, which we aren't
masking off. In most cases, it's not a problem since the
RFC1002_SESSION_MESSAGE type is 0, but when doing a RFC1002 session
establishment, the type is non-zero and that throws off the returned
length.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Tested-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>