Ensure that we check the layout pointer and validity after dereferencing
it in ff_layout_mirror_prepare_stats.
Fixes: 08e2e5bc6c ("pNFS/flexfiles: Clean up layoutstats")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>