fstests: fix flaky device name in _cleanup_flakey

fstests: fix flaky device name in _cleanup_flakey

There is no device named flakey-test, which _cleanup_flakey() currently
uses. The actual device name is stored in $FLAKEY_NAME and is set to
flakey-test.$seq. Use $FLAKEY_NAME instead.

Signed-off-by: Anand Jain <asj@kernel.org>
Fixes: 603030dee015ba ("fstests: per-test dmflakey instances")
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>