RDS/IB: rds_ib_cm_handle_connect() forgot to unlock c_cm_lock

rds_ib_cm_handle_connect() could return without unlocking the c_conn_lock if
rds_setup_qp() failed.  Rather than adding another imbalanced mutex_unlock() to
this error path we only unlock the mutex once as we exit the function, reducing
the likelyhood of making this same mistake in the future.  We remove the
previous mulitple return sites, leaving one unambigious return path.

Signed-off-by: Zach Brown <zach.brown@oracle.com>