backuptool: use correct BLK_PATH on a-only dynamic partitions devices

Change-Id: I295d39e35245e3541648369acbfe79d88ac21865

prebuilt/common/bin/backuptool.sh

@@ -137,7 +137,11 @@ find_block() {
 determine_system_mount
 
 DYNAMIC_PARTITIONS=$(getprop ro.boot.dynamic_partitions)
-BLK_PATH=$(dirname "$SYSDEV")
+if [ "$DYNAMIC_PARTITIONS" = "true" ]; then
+    BLK_PATH="/dev/block/mapper"
+else
+    BLK_PATH=$(dirname "$SYSDEV")
+fi
 
 mount_extra() {
   for partition in $@; do