fs_mgr: Copy the blk_device from start_idx to current index

If next same mount point also is logical partition, but its blk_device
is not updated yet and still is logical partition name not a mapper
device (/dev/block/dm-X) to cause mount failed.

To support the below fstab configs in fstab.postinstall.
system /postinstall ext4  ro,nosuid,nodev,noexec slotselect_other,logical
system /postinstall erofs ro,nosuid,nodev,noexec slotselect_other,logical

Error logs:
system_b: Can't open blockdev
init: [libfs_mgr]__mount(source=system_b(missing),target=/postinstall,type=erofs)=-1: No such file or directory

Bug: b/241716684
Test: After factory device reset, ensure system_b is mounted as EROFS
Change-Id: I02f20f3dfd8c42be9981915eaff88a5948482724