NFS: Close page_mkwrite() races

Follow up to Nick Piggin's patches to ensure that nfs_vm_page_mkwrite
returns with the page lock held, and sets the VM_FAULT_LOCKED flag.

See http://bugzilla.kernel.org/show_bug.cgi?id=12913

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>