generic/361: require sparse file support and fix mounting the volume.

generic/361: require sparse file support and fix mounting the volume.

The test case generic/361 fails on multiple filesystem
implementations due to two issues:

	1. A file is created on a 512 MB volume by truncating it to
	   1 GB. For this to work, the filesystem implementation must
	   support sparse files. However, the test case does not check
	   for sparse file support.
	2. A loop device, backed by the 1 GB file, is created, mkfs is
	   run on it and it is mounted. However, the mount command does
	   not specify the filesystem type. This doesn't always work
	   properly, as the kernel may pick the wrong driver if there
	   are multiple implementations for a particular filesystem,
	   or not find a driver at all in some cases.

To fix the behavior of the test case, this commit adds a requirement
for sparse file support to the test case, and explicitly specifies
the filesystem type as $FSTYP when mounting the loop device.

Signed-off-by: Ari Sundholm <ari@tuxera.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>