pNFS: pnfs_roc_drain() fix a race with open

If a process reopens the file before we can send off the CLOSE/DELEGRETURN,
then pnfs_roc_drain() may end up waiting for a new set of layout segments
that are marked as return-on-close, but haven't yet been returned.

Fix this by only waiting for those layout segments that were invalidated in
pnfs_roc().

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