wireless: wext: avoid gcc -O3 warning

wireless: wext: avoid gcc -O3 warning

After the introduction of CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3,
the wext code produces a bogus warning:

In function 'iw_handler_get_iwstats',
    inlined from 'ioctl_standard_call' at net/wireless/wext-core.c:1015:9,
    inlined from 'wireless_process_ioctl' at net/wireless/wext-core.c:935:10,
    inlined from 'wext_ioctl_dispatch.part.8' at net/wireless/wext-core.c:986:8,
    inlined from 'wext_handle_ioctl':
net/wireless/wext-core.c:671:3: error: argument 1 null where non-null expected [-Werror=nonnull]
   memcpy(extra, stats, sizeof(struct iw_statistics));
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from arch/x86/include/asm/string.h:5,
net/wireless/wext-core.c: In function 'wext_handle_ioctl':
arch/x86/include/asm/string_64.h:14:14: note: in a call to function 'memcpy' declared here

The problem is that ioctl_standard_call() sometimes calls the handler
with a NULL argument that would cause a problem for iw_handler_get_iwstats.
However, iw_handler_get_iwstats never actually gets called that way.

Marking that function as noinline avoids the warning and leads
to slightly smaller object code as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200107200741.3588770-1-arnd@arndb.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>