G_TYPE_CHECK_INSTANCE_CAST() can trigger a "-Wcast-align":

    src/core/devices/nm-device-macvlan.c: In function 'parent_changed_notify':
    /usr/include/glib-2.0/gobject/gtype.h:2421:42: error: cast increases required alignment of target type [-Werror=cast-align]
     2421 | #  define _G_TYPE_CIC(ip, gt, ct)       ((ct*) ip)
          |                                          ^
    /usr/include/glib-2.0/gobject/gtype.h:501:66: note: in expansion of macro '_G_TYPE_CIC'
      501 | #define G_TYPE_CHECK_INSTANCE_CAST(instance, g_type, c_type)    (_G_TYPE_CIC ((instance), (g_type), c_type))
          |                                                                  ^~~~~~~~~~~
    src/core/devices/nm-device-macvlan.h:13:6: note: in expansion of macro 'G_TYPE_CHECK_INSTANCE_CAST'
       13 |     (G_TYPE_CHECK_INSTANCE_CAST((obj), NM_TYPE_DEVICE_MACVLAN, NMDeviceMacvlan))
          |      ^~~~~~~~~~~~~~~~~~~~~~~~~~

Avoid that by using _NM_G_TYPE_CHECK_INSTANCE_CAST().

This can only be done for our internal usages. The public headers
of libnm are not changed.