net/mlx5: LAG, replace peer count check with direct peer lookup

net/mlx5: LAG, replace peer count check with direct peer lookup

Replace mlx5_eswitch_get_npeers() count-based check with a new
mlx5_eswitch_is_peer() function that directly verifies the peer
relationship between two eswitches.

This change prepares for SD LAG support, which is a virtual LAG that
does not have num_lag_ports capability and cannot use the count-based
peer validation.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260531113954.395443-5-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>