net: tc: improve qdisc error messages

net: tc: improve qdisc error messages

The existing error message ("Invalid qdisc name") is confusing
because it suggests that there is no qdisc with the given name. In
fact, the name does refer to a valid qdisc, but it doesn't match
the kind of an existing qdisc being modified or replaced. The
new error message provides more detail to eliminate confusion.

Signed-off-by: John Ousterhout <ouster@cs.stanford.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250116195642.2794-1-ouster@cs.stanford.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>