NFSv4/pnfs: Layoutreturn on close must handle fatal networking errors

If we have a fatal ENETDOWN or ENETUNREACH error, then the layoutreturn
on close code should also handle that as fatal, and free the layouts.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>