pNFS: Don't clear the layout return info if there are segments to return

In pnfs_clear_layoutreturn_info, ensure that we don't clear the layout
return info if there are new segments queued for return due to, for
instance, a race between a LAYOUTRETURN and a failed I/O attempt.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>