tools: ynl: fix mixing ops and notifications on one socket

tools: ynl: fix mixing ops and notifications on one socket

The multi message support loosened the connection between the request
and response handling, as we can now submit multiple requests before
we start processing responses. Passing the attr set to NlMsgs decoding
no longer makes sense (if it ever did), attr set may differ message
by messsage. Isolate the part of decoding responsible for attr-set
specific interpretation and call it once we identified the correct op.

Without this fix performing SET operation on an ethtool socket, while
being subscribed to notifications causes:

 # File "tools/net/ynl/pyynl/lib/ynl.py", line 1096, in _op
 # Exception|     return self._ops(ops)[0]
 # Exception|            ~~~~~~~~~^^^^^
 # File "tools/net/ynl/pyynl/lib/ynl.py", line 1040, in _ops
 # Exception|     nms = NlMsgs(reply, attr_space=op.attr_set)
 # Exception|                                    ^^^^^^^^^^^

The value of op we use on line 1040 is stale, it comes form the previous
loop. If a notification comes before a response we will update op to None
and the next iteration thru the loop will break with the trace above.

Fixes: 6fda63c45fe8 ("tools/net/ynl: fix cli.py --subscribe feature")
Fixes: ba8be00f68f5 ("tools/net/ynl: Add multi message support to ynl")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250618171746.1201403-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>