runtime: merge codepaths in scavengeLargest

runtime: merge codepaths in scavengeLargest

This change just makes the code in scavengeLargest easier to reason
about by reducing the number of exit points to the method. It should
still be correct either way because the condition checked at the end
(released > nbytes) will always be false if we return, but this just
makes the code a little easier to maintain.

Change-Id: If60da7696aca3fab3b5ddfc795d600d87c988238
Reviewed-on: https://go-review.googlesource.com/c/go/+/160617
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>