flexfiles/pNFS: fix NULL checks on result of ff_layout_choose_ds_for_read

Recent commit f06bedfa ("pNFS/flexfiles: don't attempt pnfs on fatal DS
errors") has changed the error return type of ff_layout_choose_ds_for_read() from
NULL to an error pointer. However, not all code paths have been updated
to match the change. Thus, some non-NULL checks will accept error pointers
as a valid return value.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: f06bedfa ("pNFS/flexfiles: don't attempt pnfs on fatal DS errors")
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>