init: fix first stage mount failure when two fstab entries have verity_loc

The previous check is incorrect because it compares the basename of
previous verity_loc with the full path of current verity_loc.

Changes it to compare the full device file path instead of just the basename
of verity_loc. This can catch the case of two different verity_loc
values with the same basename, e.g.,

  - verify=/dev/block/platform/SOC.0/by-name/metadata
  - verify=/dev/block/platform/SOC.1/by-name/metadata

Bug: 37413399
Bug: 37619597
Test: first stage mount /system and /vendor with the following fs_mgr_flags on bullhead
        - wait,verify=/dev/block/platform/soc.0/f9824900.sdhci/by-name/metadataa
Test: first stage mount /system and /vendor with different verity_loc values
      on bullhead, checks it bails out
Change-Id: I017c8bd9f0790d45e08e57df9a2878e4f62c5f9c