igb: Fix a deadlock in igb_sriov_reinit

When igb_init_interrupt_scheme in igb_sriov_reinit is failed, the lock
acquired by rtnl_lock() is not released, which causes a deadlock.
This patch adds rtnl_unlock() in error handling to fix it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>