NFSv4.1: pnfs_layout_io_set_failed must clear invalid lsegs

If pnfs_layout_io_test_failed() authorises a retry of the failed layoutgets,
we should clear the existing layout segments so that we start afresh. Do
this in pnfs_layout_io_set_failed().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>