NFS: convert nfs4 callback thread to kthread API

There's a general push to convert kernel threads to use the (much
cleaner) kthread API. This patch converts the NFSv4 callback kernel
thread to the kthread API. In addition to being generally cleaner this
also removes the dependency on signals when shutting down the thread.

Note that this patch depends on the recent patches to svc_recv() to
make it check kthread_should_stop() periodically. Those patches are
in Bruce's tree at the moment and are slated for 2.6.26 along with
the lockd conversion, so this conversion is probably also appropriate
for 2.6.26.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>