netlink: fix policy dump for int with validation callback

Recent devlink change added validation of an integer value
via NLA_POLICY_VALIDATE_FN, for sparse enums. Handle this
in policy dump. We can't extract any info out of the callback,
so report only the type.

Fixes: 429ac621 ("devlink: define enum for attr types of dynamic attributes")
Reported-by:  <syzbot+01eb26848144516e7f0a@syzkaller.appspotmail.com>
Link: https://patch.msgid.link/20250509212751.1905149-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>