nfs41: add a helper function to set layoutcommit after commit

Track lwb in nfs_commit_data so that we can use it to setup
layoutcommit in commit_done callback.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>