ethtool: add netlink based get rss support

Add netlink based support for "ethtool -x <dev> [context x]"
command by implementing ETHTOOL_MSG_RSS_GET netlink message.
This is equivalent to functionality provided via ETHTOOL_GRSSH
in ioctl path. It sends RSS table, hash key and hash function
of an interface to user space.

This patch implements existing functionality available
in ioctl path and enables addition of new RSS context
based parameters in future.

Signed-off-by: Sudheer Mogilappagari <sudheer.mogilappagari@intel.com>
Link: https://lore.kernel.org/r/20221202002555.241580-1-sudheer.mogilappagari@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>