dump: _cleanup_dump should only check the scratch fs if the test required it

_cleanup_dump always tries to check the scratch fs, even if the caller
didn't actually _require_scratch.  If a previous test wrote garbage to
the scratch device then the dump test will fail here when repair
stumbles over the garbage.

This was observed by running xfs/016 and xfs/036 in succession.  xfs/016
writes 0xc6 to the scratch device and tries to format a small log.  If
the log is too small the format fails and the test will _notrun.  The
subsequent xfs/036 will _notrun and then _cleanup_dump if no tape device
is set, at which point we try to check the scratch device and logprint
aborts due to the abnormal log size (0xc6c6c6c6).

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>