s390: Fix wrong size in memcmp (netiucv)

This error was reported by cppcheck:
drivers/s390/net/netiucv.c:568: error: Using sizeof for array given
as function argument returns the size of pointer.

sizeof(ipuser) did not result in 16 (as many programmers would have
expected) but sizeof(u8 *), so it is 4 or 8, too small here.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>