um: vector: fix missing unlock on error in vector_net_open()

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

Fixes: ad1f62ab2bd4 ("High Performance UML Vector Network Driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>