generic/740: move checking for a scratch device up

Commit 3ae8a19f97d0 ("generic/740: enable by default") moved these checks
down accidentally.  Move them back up to make the test exit gracefully
when no scratch device is provided.

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