pNFS/flexfiles: Check the layout validity in ff_layout_mirror_prepare_stats

Ensure that we check the layout pointer and validity after dereferencing
it in ff_layout_mirror_prepare_stats.

Fixes: 08e2e5bc

 ("pNFS/flexfiles: Clean up layoutstats")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>