lpm: report error when defer queue overflows

lpm: report error when defer queue overflows

Coverity complains about unchecked return value of rte_rcu_qsbr_dq_enqueue.
By default, defer queue size is big enough to hold all tbl8 groups. When
enqueue fails, return error to the user to indicate system issue.

Coverity issue: 360832
Fixes: 8a9f8564e9f9 ("lpm: implement RCU rule reclamation")

Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>