xdsclient: fix race in load report implementation

[easwars]

In clientImpl.ReportLoad(), once we get a reference to the appropriate authority object, we are supposed to increment the reference count using a call to authority.refLocked().

The documentation on this function says:

// Increments the reference count. Caller must hold parent's authorityMu.

We were releasing the mutex right before incrementing the reference count, and this was not caught by any of our unit tests. It was only caught when I ran a prober binary with the race flag enabled.

RELEASE NOTES:

• xdsclient: fix race which can happen when multiple load reporting calls are made at the same time.