devlink: fix trace format string

Including devlink.h on ARM and probably other 32-bit architectures results in
a harmless warning:

In file included from ../include/trace/define_trace.h:95:0,
                 from ../include/trace/events/devlink.h:51,
                 from ../net/core/devlink.c:30:
include/trace/events/devlink.h: In function 'trace_raw_output_devlink_hwmsg':
include/trace/events/devlink.h:42:12: error: format '%lu' expects argument of type 'long unsigned int', but argument 10 has type 'size_t {aka unsigned int}' [-Werror=format=]

The correct format string for 'size_t' is %zu, not %lu, this works on all
architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: e5224f0f ("devlink: add hardware messages tracing facility")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>