net/mlx5e: Don't include PSP in the hard MTU calculations

net/mlx5e: Don't include PSP in the hard MTU calculations

Commit [1] added the 40 bytes required by the PSP header+trailer and the
UDP header to MLX5E_ETH_HARD_MTU, which limits the device-wide max
software MTU that could be set. This is not okay, because most packets
are not PSP packets and it doesn't make sense to always reserve space
for headers which won't get added in most cases.

As it turns out, for TCP connections, PSP overhead is already taken into
account in the TCP MSS calculations via inet_csk(sk)->icsk_ext_hdr_len.
This was added in commit [2]. This means that the extra space reserved
in the hard MTU for mlx5 ends up unused and wasted.

Remove the unnecessary 40 byte reservation from hard MTU.

[1] commit e5a1861a298e ("net/mlx5e: Implement PSP Tx data path")
[2] commit e97269257fe4 ("net: psp: update the TCP MSS to reflect PSP
packet overhead")

Fixes: e5a1861a298e ("net/mlx5e: Implement PSP Tx data path")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Shahar Shitrit <shshitrit@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1765284977-1363052-10-git-send-email-tariqt@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>