ethtool: mark ETHER_FLOW as usable for Rx hash

Looks like some drivers (ena, enetc, fbnic.. there's probably more)
consider ETHER_FLOW to be legitimate target for flow hashing.
I'm not sure how intentional that is from the uAPI perspective
vs just an effect of ethtool IOCTL doing minimal input validation.
But Netlink will do strict validation, so we need to decide whether
we allow this use case or not. I don't see a strong reason against
it, and rejecting it would potentially regress a number of drivers.
So update the comments and flow_type_hashable().

Link: https://patch.msgid.link/20250708220640.2738464-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

include/uapi/linux/ethtool.h

@@ -2314,7 +2314,7 @@ enum {
 	IPV6_USER_FLOW	= 0x0e, /* spec only (usr_ip6_spec; nfc only) */
 	IPV4_FLOW	= 0x10, /* hash only */
 	IPV6_FLOW	= 0x11, /* hash only */
-	ETHER_FLOW	= 0x12, /* spec only (ether_spec) */
+	ETHER_FLOW	= 0x12, /* hash or spec (ether_spec) */
 
 	/* Used for GTP-U IPv4 and IPv6.
 	 * The format of GTP packets only includes
@@ -2371,7 +2371,7 @@ enum {
 /* Flag to enable RSS spreading of traffic matching rule (nfc only) */
 #define	FLOW_RSS	0x20000000
 
-/* L3-L4 network traffic flow hash options */
+/* L2-L4 network traffic flow hash options */
 #define	RXH_L2DA	(1 << 1)
 #define	RXH_VLAN	(1 << 2)
 #define	RXH_L3_PROTO	(1 << 3)

net/ethtool/ioctl.c

@@ -981,6 +981,7 @@ static int ethtool_rxnfc_copy_to_user(void __user *useraddr,
 static bool flow_type_hashable(u32 flow_type)
 {
 	switch (flow_type) {
+	case ETHER_FLOW:
 	case TCP_V4_FLOW:
 	case UDP_V4_FLOW:
 	case SCTP_V4_FLOW: