cryptfs: round down dm-crypt device size to crypto sector boundary

cryptfs: round down dm-crypt device size to crypto sector boundary

This is needed to make adoptable storage volumes work with a 4K crypto
sector size when the block device size is not a multiple of 4K.

It is fine to do this because the filesystem ends on a 4K boundary
anyway and doesn't use any partial block at the end.

Bug: 123375298
Test: booted device configured to use FDE with sector size 4k, ran
      'sm set-virtual-disk true' and formatted the virtual SD card as
      adoptable storage.  Then did the same but with a temporary patch
      that changed kSizeVirtualDisk to be misaligned
Change-Id: I95ee6d7dcaaa8989c674aea9988c09116e830b0c