SUNRPC: export svc_sock_update_bufs

Needed since the plan is to not have a svc_create_thread helper and to
have current users of that function just call kthread_run directly.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>