NFSv4.1: need to put_layout_hdr on _pnfs_return_layout error path

We always get a reference on the layout header and we rely on
nfs4_layoutreturn_release to put it.  If we hit an allocation error
before starting the rpc proc we bail out early without dereferncing
the layout header properly.

Signed-off-by: Benny Halevy <benny@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>