generic/231: use relative path to invoke fsx

generic/231: use relative path to invoke fsx

This test uses su to run fsx as $qa_user.  Because the shell calling
fsx 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 fsx using a relative
path, partially reverting commit 1c67e8b191fe
("config: add FSX_PROG variable").

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