runtime: fix sudog leak in syncsemrelease

Manifested as increased memory usage in a Google production system.

Not an unbounded leak, but can significantly increase the number
of sudogs allocated between garbage collections.

I checked all the other calls to acquireSudog.
This is the only one that was missing a releaseSudog.

LGTM=r, dneil
R=dneil, r
CC=golang-codereviews
https://golang.org/cl/169260043

src/runtime/sema.go

@@ -259,6 +259,7 @@ func syncsemrelease(s *syncSema, n uint32) {
 		}
 		s.tail = w
 		goparkunlock(&s.lock, "semarelease")
+		releaseSudog(w)
 	} else {
 		unlock(&s.lock)
 	}