shared/298: run xfs_db against the loop device instead of the image file

shared/298: run xfs_db against the loop device instead of the image file

xfs_db fails to properly detect the device sector size and thus segfaults
when run again an image file with 4k sector size.  While that's something
we should fix in xfs_db it will require a fair amount of refactoring of
the libxfs init code.  For now just change shared/298 to run xfs_db
against the loop device created on the image file that is used for I/O,
which feels like the right thing to do anyway to avoid cache coherency
issues.

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