use offsetof macro to make scan-build happy

use offsetof macro to make scan-build happy

scan-build from clang version 9 complains about following:

 cache.c:106:2: warning: Dereference of null pointer
        nl_list_for_each_entry_safe(obj, tmp, &cache->c_items, ce_list)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 include/netlink/list.h:83:20: note: expanded from macro 'nl_list_for_each_entry_safe'
             pos = n, n = nl_list_entry(n->member.next, typeof(*n), member))
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 include/netlink/list.h:60:2: note: expanded from macro 'nl_list_entry'
        nl_container_of(ptr, type, member)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 include/netlink/list.h:56:55: note: expanded from macro 'nl_container_of'
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
                                                      ^~~~~

Signed-off-by: Petr Štetiar <ynezz@true.cz>