RDMA/mana_ib: Prefer struct_size over open coded arithmetic

RDMA/mana_ib: Prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "req" variable is a pointer to "struct mana_cfg_rx_steer_req_v2"
and this structure ends in a flexible array:

struct mana_cfg_rx_steer_req_v2 {
	[...]
        mana_handle_t indir_tab[] __counted_by(num_indir_entries);
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + size * count" in
the kzalloc() function.

Moreover, use the "offsetof" helper to get the indirect table offset
instead of the "sizeof" operator and avoid the open-coded arithmetic in
pointers using the new flex member. This new structure member also allow
us to remove the "req_indir_tab" variable since it is no longer needed.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
Link: https://github.com/KSPP/linux/issues/160
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Link: https://lore.kernel.org/r/AS8PR02MB72375EB06EE1A84A67BE722E8B022@AS8PR02MB7237.eurprd02.prod.outlook.com
Reviewed-by: Long Li <longli@microsoft.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>