net/af_iucv: Use struct_group() to zero struct iucv_sock region

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add struct_group() to mark the region of struct iucv_sock that gets
initialized to zero. Avoid the future warning:

In function 'fortify_memset_chk',
    inlined from 'iucv_sock_alloc' at net/iucv/af_iucv.c:476:2:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Acked-by: Karsten Graul <kgraul@linux.ibm.com>
Link: https://lore.kernel.org/lkml/19ff61a0-0cda-6000-ce56-dc6b367c00d6@linux.ibm.com/


Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>