runtime: make m.nextwaitm an muintptr

This field is really a *m (modulo its bottom bit). Change it from
uintptr to muintptr to document this fact.

Change-Id: I2d181a955ef1d2c1a268edf20091b440d85726c9
Reviewed-on: https://go-review.googlesource.com/46034
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/lock_sema.go

@@ -71,7 +71,7 @@ Loop:
 			// for this lock, chained through m->nextwaitm.
 			// Queue this M.
 			for {
-				gp.m.nextwaitm = v &^ locked
+				gp.m.nextwaitm = muintptr(v &^ locked)
 				if atomic.Casuintptr(&l.key, v, uintptr(unsafe.Pointer(gp.m))|locked) {
 					break
 				}
@@ -103,8 +103,8 @@ func unlock(l *mutex) {
 		} else {
 			// Other M's are waiting for the lock.
 			// Dequeue an M.
-			mp = (*m)(unsafe.Pointer(v &^ locked))
-			if atomic.Casuintptr(&l.key, v, mp.nextwaitm) {
+			mp = muintptr(v &^ locked).ptr()
+			if atomic.Casuintptr(&l.key, v, uintptr(mp.nextwaitm)) {
 				// Dequeued an M.  Wake it.
 				semawakeup(mp)
 				break

src/runtime/runtime2.go

@@ -432,7 +432,7 @@ type m struct {
 	fflag         uint32         // floating point compare flags
 	lockedExt     uint32         // tracking for external LockOSThread
 	lockedInt     uint32         // tracking for internal lockOSThread
-	nextwaitm     uintptr        // next m waiting for lock
+	nextwaitm     muintptr       // next m waiting for lock
 	waitunlockf   unsafe.Pointer // todo go func(*g, unsafe.pointer) bool
 	waitlock      unsafe.Pointer
 	waittraceev   byte