net/mlx5: Refactor fragmented buffer struct fields and init flow

Take struct mlx5_frag_buf out of mlx5_frag_buf_ctrl, as it is not
needed to manage and control the datapath of the fragmented buffers API.

struct mlx5_frag_buf contains control info to manage the allocation
and de-allocation of the fragmented buffer.
Its fields are not relevant for datapath, so here I take them out of the
struct mlx5_frag_buf_ctrl, except for the fragments array itself.

In addition, modified mlx5_fill_fbc to initialise the frags pointers
as well. This implies that the buffer must be allocated before the
function is called.

A set of type-specific *_get_byte_size() functions are replaced by
a generic one.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>