xds/balancer/priority: Unlock mutex before returning

[arjan-bal]

Release the mutex that is locked a couple of lines above.

grpc-go/xds/internal/balancer/priority/balancer.go

Lines 271 to 274 in d27ddb5

	b.mu.Lock()
	if b.done.HasFired() {
	return
	}

This could subsequent calls to UpdateClientConnState to get stuck.

RELEASE NOTES: None

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.33%. Comparing base (bdd707e) to head (040de9b).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7417      +/-   ##
==========================================
- Coverage   81.42%   81.33%   -0.09%     
==========================================
  Files         348      350       +2     
  Lines       26744    26846     +102     
==========================================
+ Hits        21775    21834      +59     
- Misses       3779     3809      +30     
- Partials     1190     1203      +13     

Files	Coverage Δ
xds/internal/balancer/priority/balancer.go	87.82% <0.00%> (-0.78%)	⬇️

... and 32 files with indirect coverage changes

[dfawley]

This could subsequent calls to UpdateClientConnState to get stuck.

Calling a Closed Balancer would be illegal, and isn't something we should ever do anywhere for any reason.