sfc: fix potential stack corruption from running past stat bitmask

On 32-bit systems, mask is only an array of 3 longs, not 4, so don't try
to write to mask[3].
Also include build-time checks in case the size of the bitmask changes.

Fixes: 3c36a2ad ("sfc: display vadaptor statistics for all interfaces")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>