ethtool: refactor checking max channels

ethtool: refactor checking max channels

Currently ethtool_set_channel calls separate functions to check whether
the new channel number violates rss configuration or flow steering
configuration.

Very soon we need to check whether the new channel number violates
memory provider configuration as well.

To do all 3 checks cleanly, add a wrapper around
ethtool_get_max_rxnfc_channel() and ethtool_get_max_rxfh_channel(),
which does both checks. We can later extend this wrapper to add the
memory provider check in one place.

Note that in the current code, we put a descriptive genl error message
when we run into issues. To preserve the error message, we pass the
genl_info* to the common helper. The ioctl calls can pass NULL instead.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20240808205345.2141858-1-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>