vold: meta encryption: fix /dev/block/by-name/userdata to dm-3

vold: meta encryption: fix /dev/block/by-name/userdata to dm-3

This fixes F2FS GC failure in idle-maint.
07-28 18:25:54.838   603 11187 D vold    : idle maintenance started
07-28 18:25:54.846   603 11187 D vold    : Start GC on /sys/fs/f2fs/sda21
07-28 18:25:54.847   603 11187 W vold    : Set discard gralunarity failed on/sys/fs/f2fs/sda21: No such file or directory
07-28 18:25:54.847   603 11187 W vold    : Start GC failed on /sys/fs/f2fs/sda21: No such file or directory

Bug: 111953875
Change-Id: I1d10802121d5641cf9ba780fee249affd2cf6ffe
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>