net/mlx4: optimize Tx multi-segment case

net/mlx4: optimize Tx multi-segment case

mlx4 Tx block can handle up to 4 data segments or control segment + up
to 3 data segments. The first data segment in each not first Tx block
must validate Tx queue wraparound and must use IO memory barrier before
writing the byte count.

The previous multi-segment code used "for" loop to iterate over all
packet segments and separated first Tx block data case by "if"
statements.

Use switch case and unconditional branches instead of "for" loop can
optimize the case and prevents the unnecessary checks for each data
segment; This hints to compiler to create optimized jump table.

Optimize this case by switch case and unconditional branches usage.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>