NFSv4/pnfs: Reset the layout state after a layoutreturn

If there are still layout segments in the layout plh_return_lsegs list
after a layout return, we should be resetting the state to ensure they
eventually get returned as well.

Fixes: 68f74479 ("pNFS: Do not free layout segments that are marked for return")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>