cmd/go/internal/modfetch: add Unlock before return in checkModSum

In cmd/go/internal/modfetch/fetch.go,
`checkModSum()` forgets Unlock before return, which may lead to deadlock.
876c1feb7d/src/cmd/go/internal/modfetch/fetch.go (L514-L520)
The fix is to add `goSum.mu.Unlock()` before return.

Change-Id: I855b1c1bc00aeada2c1e84aabb5328f02823007d
GitHub-Last-Rev: afeb3763dd
GitHub-Pull-Request: golang/go#38563
Reviewed-on: https://go-review.googlesource.com/c/go/+/229219
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/fetch.go

@@ -514,6 +514,7 @@ func checkModSum(mod module.Version, h string) error {
 	goSum.mu.Lock()
 	inited, err := initGoSum()
 	if err != nil {
+		goSum.mu.Unlock()
 		return err
 	}
 	done := inited && haveModSumLocked(mod, h)