module: Try to explicitly mount dm-linear block devices

The source field for dm-linear block devices in the recovery's fstab is
not an actual file path.

Fixes: #338

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

app/magisk/update-binary

@@ -14,13 +14,18 @@ if [ -f /sbin/recovery ] || [ -f /system/bin/recovery ]; then
     set -exu
 
     ui_print 'Mounting system'
-    if mount /system_root; then
+
+    slot=$(getprop ro.boot.slot_suffix)
+
+    if mount /system_root || mount /dev/block/mapper/system_root"${slot}" /system_root; then
         mount -o remount,rw /system_root
         root_dir=/system_root
-    else
-        mount /system
+    elif mount /system || mount /dev/block/mapper/system"${slot}" /system; then
         mount -o remount,rw /system
         root_dir=/
+    else
+        ui_print 'Failed to mount system partition'
+        exit 1
     fi
 
     ui_print 'Extracting files'