NFSv4: It is not safe to dereference lsp->ls_state in release_lockowner

It is quite possible for the release_lockowner RPC call to race with the
close RPC call, in which case, we cannot dereference lsp->ls_state in
order to find the nfs_server.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>