bnxt: avoid overflow in bnxt_get_nvram_directory()

The value of an arithmetic expression is subject
of possible overflow due to a failure to cast operands to a larger data
type before performing arithmetic. Used macro for multiplication instead
operator for avoiding overflow.

Found by Security Code and Linux Verification
Center (linuxtesting.org) with SVACE.
Signed-off-by: Maxim Korotkov <korotkov.maxim.s@gmail.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230309174347.3515-1-korotkov.maxim.s@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c

@@ -2862,7 +2862,7 @@ static int bnxt_get_nvram_directory(struct net_device *dev, u32 len, u8 *data)
 	if (rc)
 		return rc;
 
-	buflen = dir_entries * entry_length;
+	buflen = mul_u32_u32(dir_entries, entry_length);
 	buf = hwrm_req_dma_slice(bp, req, buflen, &dma_handle);
 	if (!buf) {
 		hwrm_req_drop(bp, req);