[CIFS] Do not allow signals in cifs_demultiplex_thread

Switch from send_sig to force_sig and do not allow signal for this
background thread (the signal is needed to wakeup the thread when
blocked in the network stack).

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