Create sysfs_dm label.

Prior to this CL, /sys/devices/virtual/block/dm-X was using the generic
sysfs label. This CL creates sysfs_dm label and grants the following
accesses:
 - update_verifier to read sysfs_dm dir and file at
   /sys/devices/virtual/block/dm-X.
 - vold to write sysfs_dm.

Bug: 63440407
Test: update_verifier successfully triggers blocks verification and
      marks a sucessful boot;
Test: No sysfs_dm related denials on sailfish.
Change-Id: I6349412707800f1bd3a2fb94d4fe505558400c95