pNFS/flexfiles: Avoid spurious layout returns in ff_layout_choose_ds_for_read

The callers of ff_layout_choose_ds_for_read() should decide whether or
not they want to return the layout on error. Sometimes, we may just want
to retry from the beginning.

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