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: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/ef10.c

@@ -1517,13 +1517,14 @@ static void efx_ef10_get_stat_mask(struct efx_nic *efx, unsigned long *mask)
 	}
 
 #if BITS_PER_LONG == 64
+	BUILD_BUG_ON(BITS_TO_LONGS(EF10_STAT_COUNT) != 2);
 	mask[0] = raw_mask[0];
 	mask[1] = raw_mask[1];
 #else
+	BUILD_BUG_ON(BITS_TO_LONGS(EF10_STAT_COUNT) != 3);
 	mask[0] = raw_mask[0] & 0xffffffff;
 	mask[1] = raw_mask[0] >> 32;
 	mask[2] = raw_mask[1] & 0xffffffff;
-	mask[3] = raw_mask[1] >> 32;
 #endif
 }