IB/cm: Fix missing unlock on error in cm_init_qp_rtr_attr()

Add the missing unlock before return from function cm_init_qp_rtr_attr()
in the error handling case.

Fixes: dd5f03be ("IB/core: Ethernet L2 attributes in verbs/cm structures")
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Roland Dreier <roland@purestorage.com>