nfsv4: release file state only after successful LayoutReturn

in a situation when layout return takes log time, NFS server will reply
with ERR_DELAY. This will trigger retry on the client side. If
file state is already invalidated, then client will get BAD_STATEID
and propagate error to the application.

Acked-by: Gerd Behrmann
Target: master, 0.9, 0.8, 0.7