When the blocked field was first introduced back in
https://golang.org/cl/61250043 the scheduler trace code incorrectly used
m->blocked instead of mp->blocked.  That has carried through the
conversion to Go.  This CL fixes it.

Change-Id: Id81907b625221895aa5c85b9853f7c185efd8f4b
Reviewed-on: https://go-review.googlesource.com/24571Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>