Fix JFFS2 sync silent failure

JFFS2 does not appear to set s_bdi anywhere.  And as of 32a88aa1,
__sync_filesystem() will return 0 if s_bdi is not set.  As a result,
sync_fs() is never called for jffs2 and whatever remains in the wbuf
will not make it to the device.

Fix that up by assigning the mtd bdi.

Signed-off-by: Jörn Engel <joern@logfs.org>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>