cifs: advertise the right receive buffer size to the server

Currently, we mirror the same size back to the server that it sends us.
That makes little sense. Instead we should be sending the server the
maximum buffer size that we can handle -- CIFSMaxBufSize minus the
4 byte RFC1001 header.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>