net/mlx5e: RX, Take shared info fragment addition into a function

Introduce mlx5e_add_skb_shared_info_frag(), a function dedicated for
adding a fragment into a struct skb_shared_info object.

Use it in the Legacy RQ flow. Similar usage will be added in a
downstream patch by the corresponding Striding RQ flow.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>