pnfsblock: fix size of upcall message

Make the status field explicitly 32 bits.  "...it's unlikely that the kernel
and userspace would differ on the size of an int here, but it might be a
good idea to go ahead and make that explicitly 32 bits in case we end up
dealing with more exotic arches at some point in the future."

Suggested-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Cc: stable@kernel.org [3.0]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>