netlink: specs: define input-xfrm enum in the spec

Help YNL decode the values for input-xfrm by defining
the possible values in the spec. Don't define "no change"
as it's an IOCTL artifact with no use in Netlink.

With this change on mlx5 input-xfrm gets decoded:

 # ynl --family ethtool --dump rss-get
 [{'header': {'dev-index': 2, 'dev-name': 'eth0'},
   'hfunc': 1,
   'hkey': b'V\xa8\xf9\x9 ...',
   'indir': [0, 1, ... ],
   'input-xfrm': {'sym-or-xor'},                         <<<
   'flow-hash': {'ah4': {'ip-dst', 'ip-src'},
                 'ah6': {'ip-dst', 'ip-src'},
                 'esp4': {'ip-dst', 'ip-src'},
                 'esp6': {'ip-dst', 'ip-src'},
                 'ip4': {'ip-dst', 'ip-src'},
                 'ip6': {'ip-dst', 'ip-src'},
                 'tcp4': {'l4-b-0-1', 'ip-dst', 'l4-b-2-3', 'ip-src'},
                 'tcp6': {'l4-b-0-1', 'ip-dst', 'l4-b-2-3', 'ip-src'},
                 'udp4': {'l4-b-0-1', 'ip-dst', 'l4-b-2-3', 'ip-src'},
                 'udp6': {'l4-b-0-1', 'ip-dst', 'l4-b-2-3', 'ip-src'}}
 }]

Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250716000331.1378807-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/ethtool.yaml

@@ -158,6 +158,26 @@ definitions:
       -
         name: pse-event-sw-pw-control-error
         doc: PSE faced an error managing the power control from software
+  -
+    name: input-xfrm
+    doc: RSS hash function transformations.
+    type: flags
+    enum-name:
+    name-prefix: rxh-xfrm-
+    header: linux/ethtool.h
+    entries:
+      -
+        name: sym-xor
+        doc: >-
+          XOR the corresponding source and destination fields of each specified
+          protocol. Both copies of the XOR'ed fields are fed into the RSS and
+          RXHASH calculation. Note that this XORing reduces the input set
+          entropy and could be exploited to reduce the RSS queue spread.
+      -
+        name: sym-or-xor
+        doc: >-
+          Similar to SYM_XOR, except that one copy of the XOR'ed fields is
+          replaced by an OR of the same fields.
   -
     name: rxfh-fields
     name-prefix: rxh-
@@ -1621,6 +1641,7 @@ attribute-sets:
       -
         name: input-xfrm
         type: u32
+        enum: input-xfrm
       -
         name: start-context
         type: u32