module: return bool from within_module*()

The within_module*() functions return only true or false. Let's use bool as
the return type.

Note that it should not change kABI because these are inline functions.

Signed-off-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>