ethtool: rss: support creating contexts via Netlink

ethtool: rss: support creating contexts via Netlink

Support creating contexts via Netlink. Setting flow hashing
fields on the new context is not supported at this stage,
it can be added later.

An empty indirection table is not supported. This is a carry
over from the IOCTL interface where empty indirection table
meant delete. We can repurpose empty indirection table in
Netlink but for now to avoid confusion reject it using the
policy.

Support letting user choose the ID for the new context. This was
not possible in IOCTL since the context ID field for the create
action had to be set to the ETH_RXFH_CONTEXT_ALLOC magic value.

Link: https://patch.msgid.link/20250717234343.2328602-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>