runtime: Plan 9 - prevent preemption by GC while exiting

On Plan 9, there's no "kill all threads" system call, so exit is done
by sending a "go: exit" note to each OS process.  If concurrent GC
occurs during this loop, deadlock sometimes results.  Prevent this by
incrementing m.locks before sending notes.

Change-Id: I31aa15134ff6e42d9a82f9f8a308620b3ad1b1b1
Reviewed-on: https://go-review.googlesource.com/20477Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os1_plan9.go

@@ -151,6 +151,7 @@ var goexits = []byte("go: exit ")
 
 func goexitsall(status *byte) {
 	var buf [_ERRMAX]byte
+	getg().m.locks++
 	n := copy(buf[:], goexits)
 	n = copy(buf[n:], gostringnocopy(status))
 	pid := getpid()
@@ -159,6 +160,7 @@ func goexitsall(status *byte) {
 			postnote(mp.procid, buf[:])
 		}
 	}
+	getg().m.locks--
 }
 
 var procdir = []byte("/proc/")