net: sched: sch_api: set an error msg when qdisc_alloc_handle() fails

This patch sets an error message in extack when the number of qdisc
handles exceeds the maximum. Also the error-code ENOSPC is more
appropriate than ENOMEM in this situation.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reported-by: Li Shuang <shuali@redhat.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>