net: ethtool: fix compat with old RSS context API

net: ethtool: fix compat with old RSS context API

Device driver gets access to rxfh_dev, while rxfh is just a local
copy of user space params. We need to check what RSS context ID
driver assigned in rxfh_dev, not rxfh.

Using rxfh leads to trying to store all contexts at index 0xffffffff.
From the user perspective it leads to "driver chose duplicate ID"
warnings when second context is added and inability to access any
contexts even tho they were successfully created - xa_load() for
the actual context ID will return NULL, and syscall will return -ENOENT.

Looks like a rebasing mistake, since rxfh_dev was added relatively
recently by commit fb6e30a72539 ("net: ethtool: pass a pointer to
parameters to get/set_rxfh ethtool ops").

Fixes: eac9122f0c41 ("net: ethtool: record custom RSS contexts in the XArray")
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20240702164157.4018425-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>