IB/core: Take sizeof the correct pointer when calling kmalloc()

When allocating out_mad in show_pma_counter(), take sizeof *out_mad
instead of sizeof *in_mad.  It is true that today the type of in_mad
and out_mad are the same, but this patch will give us a cleaner code.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>