pNFS/flexfiles: Avoid spurious layout returns in ff_layout_choose_ds_for_read

[ Upstream commit 9bfffea3 ]

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>
Stable-dep-of: f06bedfa ("pNFS/flexfiles: don't attempt pnfs on fatal DS errors")
Signed-off-by: Sasha Levin <sashal@kernel.org>