nfs4.1: prevent race that allowed use of freed layout in _pnfs_return_layout

mark_matching_lsegs_invalid could put the last ref to the layout, so
the get_layout_hdr needs to be called first.

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