generic/233: use relative path to invoke fsstress

generic/233: use relative path to invoke fsstress

This test uses su to run fsstress as $qa_user.  Because the shell calling
fsstress is run as $qa_user it might not be able to access the xfstests
or parent directory which might only be accessible to the user xfstests
is run as (e.g. root).  Switch back to calling fsstress using a relative
path, partially reverting the effects of commit c475ff6ff6d7
("common/config: add $here to FSSTRESS_PROG").

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>