net/mlx5e: RX, Generalise function of SKB frag addition

Rename it and pass truesize as an extra argument, as it will be used also
in Legacy RQ in a downstream patch.

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