nfs: kill renewd before clearing client minor version

renewd should be synchronously killed before we destroy the session in
nfs4_clear_minor_version

Signed-off-by: Alexandros Batsakis <batsakis@netapp.com>
[Trond.Myklebust@netapp.com: clean up to remove 'unused function
warning when !CONFIG_NFS_V4]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>