udev: use DM ignore device flag only for DM devices

For example do not exclude (from exposing to qvm-block) loop devices
with this flag set, otherwise `qvm-block -l` would not list just
attached disk image files.

Fixes QubesOS/qubes-issues#2453

udev/udev-qubes-block.rules

@@ -14,7 +14,7 @@ ENV{MAJOR}=="202", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="snapshot-*", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="origin-*", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="", GOTO="qubes_block_end"
-ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}=="1", GOTO="qubes_block_end"
+KERNEL=="dm-*", ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}=="1", GOTO="qubes_block_end"
 
 IMPORT{db}="QUBES_EXPOSED"
 ACTION=="add", IMPORT{program}="/usr/lib/qubes/udev-block-add-change"