net/mlx5e: Build SKB with exact frag_size

Build the SKB over the receive packet instead of the
whole page. Getting the SKB's linear data and shared_info
closer improves locality.
In addition, this opens up the possibility to make use of
other parts of the page in the downstream page-reuse patch.

Fixes: 1bfecfca ("net/mlx5e: Build RX SKB on demand")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>