ANDROID: dm-user: Drop additional reference

kref_init already takes one reference during
initialization. Additional reference will cause
memory leak when dm-user target is destroyed.

Bug: 188362111
Test: OTA - verify ref count dropping to 0
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I0b25135371daefdaf2841e9ff5cdfd6aaa389f3b