ethtool: rss: support removing contexts via Netlink

Implement removing additional RSS contexts via Netlink.
Technically it'd be possible to shoehorn the delete operation
into ethnl_request_ops-compatible handler. The code ends
up longer than open coded version, and I think we'll need
a custom way of sending notifications at some stage (if we
allow tying the context lifetime to the netlink socket, in
the future).

Link: https://patch.msgid.link/20250717234343.2328602-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/ethtool_netlink_generated.h

@@ -842,6 +842,7 @@ enum {
 	ETHTOOL_MSG_TSCONFIG_SET,
 	ETHTOOL_MSG_RSS_SET,
 	ETHTOOL_MSG_RSS_CREATE_ACT,
+	ETHTOOL_MSG_RSS_DELETE_ACT,
 
 	__ETHTOOL_MSG_USER_CNT,
 	ETHTOOL_MSG_USER_MAX = (__ETHTOOL_MSG_USER_CNT - 1)
@@ -901,6 +902,7 @@ enum {
 	ETHTOOL_MSG_RSS_NTF,
 	ETHTOOL_MSG_RSS_CREATE_ACT_REPLY,
 	ETHTOOL_MSG_RSS_CREATE_NTF,
+	ETHTOOL_MSG_RSS_DELETE_NTF,
 
 	__ETHTOOL_MSG_KERNEL_CNT,
 	ETHTOOL_MSG_KERNEL_MAX = (__ETHTOOL_MSG_KERNEL_CNT - 1)