NFSv4.1/pnfs: Fix setting of layoutcommit last write byte

If the NFS_INO_LAYOUTCOMMIT flag was unset, then we _must_ ensure that
we also reset the last write byte (lwb) for that layout. The current
code depends on us clearing the lwb when we clear NFS_INO_LAYOUTCOMMIT,
which is not the case when we call pnfs_clear_layoutcommit().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>