runtime: fix data race in Plan9 sysalloc Add mutex to protect brk limit. Add mstats.sys update.
runtime: fix data race in Plan9 sysalloc
Add mutex to protect brk limit.
Add mstats.sys update.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4762045

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rdp/rogerpack2005-golang,npe9/noahevans-go9,webfd/go-zh,npe9/noahevans-go9,sanjosh/sanjos100-tipc,npe9/noahevans-go9,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,webfd/go-zh,Triskite/willstone-goclone,d0f/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,npe9/noahevans-go9,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,webfd/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,npe9/noahevans-go9,bryanxu/go-zh,rdp/rogerpack2005-golang,npe9/noahevans-go9,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,npe9/noahevans-go9,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,d0f/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,Triskite/willstone-goclone,scirelli/scirelli-go,Triskite/willstone-goclone,scirelli/scirelli-go,bryanxu/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,npe9/noahevans-go9,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,scirelli/scirelli-go,bryanxu/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof