fix #7183: vzdump: handle mount points with a relative path

The path of a mount point does not need to start with a slash, for
example a mount point configured with

  mp0: tank:subvol-999-disk-0,mp=data,size=1G,backup=1

gets mounted at /data in the container when it starts. The backup code
however builds its paths by appending the path of the mount point to
the root directory, which only works with a leading slash. So such a
mount point broke all three backup modes, as it ended up as
'/mnt/vzsnap0/.data' instead of '/mnt/vzsnap0/./data' in snapshot and
stop mode, and as '/proc/<PID>/root//.data' in suspend mode.

Add the missing leading slash when collecting the mount points for the
backup, so that all paths built from it are correct. This also anchors
the exclude pattern of such a mount point if it is excluded from the
backup, which otherwise matched every file or directory with the same
name in suspend mode.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/VZDump/LXC.pm

@@ -157,6 +157,10 @@ sub prepare {
 
         next if !$volid || !$mount;
 
+        # the path of a mount point is always relative to the root of the container, but it
+        # does not need to start with a slash, which all the paths built below rely on
+        $mount = $volume_config->{mp} = "/$mount" if $mount !~ m!^/!;
+
         if (!$included) {
             my $type = $volume_config->{type};
             push @$exclude_dirs, $mount;