autotest: attempt to clean up Ceph mount

This can be left mounted from a previous build.  Try to clean it up
before starting up Ceph again.

Change-Id: I6273c2bbff53163177ea709dc4f4db4937a00a31
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/406627
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

scripts/ceph/start.sh

@@ -15,6 +15,10 @@ mnt_dir=${base_dir}/mnt
 dev_backend=/dev/ceph
 image=/var/tmp/ceph_raw.img
 dev=/dev/loop200
+
+umount $dev || true
+losetup -d $dev_backend || true
+
 # partition osd
 if [ -d $base_dir ]; then
 	rm -rf $base_dir