net: mana: Use struct_size() helper in mana_gd_create_dma_region()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/net/ethernet/microsoft/mana/gdma_main.c:677:24: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>