transport: protect field access with transport to avoid possible data race

[yanke-xu]

Fixed inconsistency and also potential data race in weave/vendor/k8s.io/client-go/transport/cert_rotation.go:
c.clientCert is read/written 4 times in weave/vendor/k8s.io/client-go/transport/cert_rotation.go; 3 out of 4 times it is protected by c.certMtx; 1 out of 4 times it is read without a Lock, which is in func loadClientCert() on L75.
A data race may happen when loadClientCert() and loadClientCert() are called in parallel.

For example:
for {
......
go loadClientCert()
......
}

In order to avoid potential data race here, I use c.certMtx.RLock(); defer c.certMtx.RUnlock() to make sure that all usages of items is in critical section.