net: ethtool: prevent flow steering to RSS contexts which don't exist

Since commit 42dc431f ("ethtool: rss: prevent rss ctx deletion
when in use") we prevent removal of RSS contexts pointed to by
existing flow rules. Core should also prevent creation of rules
which point to RSS context which don't exist in the first place.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250206235334.1425329-2-kuba@kernel.org


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