crypto/mlx5: fix size of UMR WQE

crypto/mlx5: fix size of UMR WQE

The size of the UMR WQE allocated object is decided by a sizof
operation on the struct, however since the struct contains
a union of flexible array members this sizeof results can differ
between compilers.

GCC for example treats the union as 0 sized, MSVC adds a padding
of 16Bits.

To resolve the ambiguity the allocation size will be calculated
by the sizes of the members excluding the flexible union.

Fixes: a1978aa23bf4 ("crypto/mlx5: add maximum segments configuration")
Cc: stable@dpdk.org
Signed-off-by: Tal Shnaiderman <talshn@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>