module: Try to explicitly mount common system partition 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

@@ -17,10 +17,27 @@ if [ -f /sbin/recovery ] || [ -f /system/bin/recovery ]; then
 
     slot=$(getprop ro.boot.slot_suffix)
 
-    if mount /system_root || mount /dev/block/mapper/system_root"${slot}" /system_root; then
+    try_mount_any() {
+        local target="${1}" path
+        shift
+
+        mount "${target}" && return 0
+
+        for path in "${@}"; do
+            mount "${path}" "${target}" && return 0
+        done
+
+        return 1
+    }
+
+    if try_mount_any /system_root \
+        /dev/block/mapper/system_root"${slot}" \
+        /dev/block/bootdevice/by-name/system_root; then
         mount -o remount,rw /system_root
         root_dir=/system_root
-    elif mount /system || mount /dev/block/mapper/system"${slot}" /system; then
+    elif try_mount_any /system \
+        /dev/block/mapper/system"${slot}" \
+        /dev/block/bootdevice/by-name/system; then
         mount -o remount,rw /system
         root_dir=/
     else