cifs: fix the type of cifs_demultiplex_thread()

... and get rid of a bogus typecast, while we are at it; it's not
just that we want a function returning int and not void, but cast
to pointer to function taking void * and returning void would be
(void (*)(void *)) and not (void *)(void *), TYVM...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>