ENOTSUPP is documented as "should never be seen by user programs",
and thus not exposed in <errno.h>, and thus applications cannot safely
check against it (they get "Unknown error 524" as strerror). We should
rather return the well-known -EOPNOTSUPP.

This is similar to 2230a7ef ("drop_monitor: Use correct error
code") and 4a5cdc60 ("net/tls: Fix return values to avoid
ENOTSUPP"), which did not seem to cause problems.
Signed-off-by: Samuel Thibault <samuel.thibault@labri.fr>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20220307223126.djzvg44v2o2jkjsx@beginSigned-off-by: Jakub Kicinski <kuba@kernel.org>