fstests: don't use directory stacks

fstests: don't use directory stacks

Using bash directory stacking (pushd, popd, etc) seems to be
somewhat unreliable. I've been seeing occasional random failures
from both pushd and popd commands that cause the test to fail, and
there does not appear to be any reason for the failures occurring.

Rather than wasting time chasing ghosts, just get rid of the
directory stacking altogether.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>