net/sched: act_connmark: handle errno on tcf_idr_check_alloc

Smatch reports that 'ci' can be used uninitialized.
The current code ignores errno coming from tcf_idr_check_alloc, which
will lead to the incorrect usage of 'ci'. Handle the errno as it should.

Fixes: 288864ef ("net/sched: act_connmark: transition to percpu stats and rcu")
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>