ueventd: Put dm-user devices in a folder

When a device-mapper table contains a "user" entry, a corresponding
dm-user misc control device is created. The devices are put into a
separate folder by default, which currently confuses ueventd, and we
wind up with paths like:

    /dev/dm-user!blah

Special case these devices so they wind up as:

    /dev/dm-user/blah

Test: dmctl create blah user 0 100 test-device
      /dev/dm-user/test-device exists
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: I313db07c3400f14f3ed0ffa20fdac2ac3e34b6d3