runtime: fix accounting race in printlock

It could happen that mp.printlock++ happens, then on entry to lock,
the goroutine is preempted and then rescheduled onto another m
for the actual call to lock. Now the lock and the printlock++ have
happened on different m's. This can lead to printlock not being
unlocked, which either gives a printing deadlock or a crash when
the goroutine reschedules, because m.locks > 0.

Change-Id: Ib0c08740e1b53de3a93f7ebf9b05f3dceff48b9f
Reviewed-on: https://go-review.googlesource.com/2819


Reviewed-by: Rick Hudson <rlh@golang.org>