cifs: Add CONFIG_CIFS_DEBUG and rename use of CIFS_DEBUG

This can reduce the size of the module by ~120KB which
could be useful for embedded systems.

$ size fs/cifs/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 388567	  34459	 100440	 523466	  7fcca	fs/cifs/built-in.o.new
 495970	  34599	 117904	 648473	  9e519	fs/cifs/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>