init: mount_handler: detect main block device more reliably

Current code is not portable beyond SCSI devices (e.g., UFS).
For example, eMMC and NVMe devices fail due to their extra postfix.

Change its logic to rewind each character until "queue" directory appears.

Test: Confirm md0p1, sda20, nvme0n1p3, mmcblk0p3 are all handled well.
Change-Id: I585ccf2d4a72f6ef8ecb203acdd72a1e32d3e749
Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>