include/linux/list_bl.h: use bool instead of int for boolean functions

hlist_bl_unhashed() and hlist_bl_empty() are all boolean functions, so
return bool instead of int.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>