runtime: change fixalloc's chunk field to unsafe.Pointer

It's never used as a *byte anyway, so might as well just make it an
unsafe.Pointer instead.

Change-Id: I68ee418781ab2fc574eeac0498f2515b5561b7a8
Reviewed-on: https://go-review.googlesource.com/16175
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/mfixalloc.go

@@ -23,7 +23,7 @@ type fixalloc struct {
 	first  func(arg, p unsafe.Pointer) // called first time p is returned
 	arg    unsafe.Pointer
 	list   *mlink
-	chunk  *byte
+	chunk  unsafe.Pointer
 	nchunk uint32
 	inuse  uintptr // in-use bytes now
 	stat   *uint64
@@ -64,15 +64,15 @@ func fixAlloc_Alloc(f *fixalloc) unsafe.Pointer {
 		return v
 	}
 	if uintptr(f.nchunk) < f.size {
-		f.chunk = (*uint8)(persistentalloc(_FixAllocChunk, 0, f.stat))
+		f.chunk = persistentalloc(_FixAllocChunk, 0, f.stat)
 		f.nchunk = _FixAllocChunk
 	}
 
-	v := unsafe.Pointer(f.chunk)
+	v := f.chunk
 	if f.first != nil {
 		f.first(f.arg, v)
 	}
-	f.chunk = (*byte)(add(unsafe.Pointer(f.chunk), f.size))
+	f.chunk = add(f.chunk, f.size)
 	f.nchunk -= uint32(f.size)
 	f.inuse += f.size
 	return v