common/ext4: provide custom ext4 scratch fs options

common/ext4: provide custom ext4 scratch fs options

Create a _scratch_options backend for ext* so that we can inject
pathnames to external log devices into the scratch fs mount options.
This enables common/dm* to install block device filters, e.g. dm-error
for stress testing.

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