net: ethtool: Don't check if RSS context exists in case of context 0

Context 0 (default context) always exists, there is no need to check
whether it exists or not when adding a flow steering rule.

The existing check fails when creating a flow steering rule for context
0 as it is not stored in the rss_ctx xarray.

For example:
$ ethtool --config-ntuple eth2 flow-type tcp4 dst-ip 194.237.147.23 dst-port 19983 context 0 loc 618
rmgr: Cannot insert RX class rule: Invalid argument
Cannot insert classification rule

An example usecase for this could be:
- A high-priority rule (loc 0) directing specific port traffic to
  context 0.
- A low-priority rule (loc 1) directing all other TCP traffic to context
  1.

This is a user-visible regression that was caught in our testing
environment, it was not reported by a user yet.

Fixes: de7f7582 ("net: ethtool: prevent flow steering to RSS contexts which don't exist")
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20250612071958.1696361-2-gal@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>