runtime: fix sudog leak in syncsemrelease

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