Although btrfs-image will dump log tree, we will modify the restored
image if it's not a multi-device restore.

In that case, since log tree blocks are not recorded in extent tree,
extent allocator will try to re-use the tree blocks belonging to log
trees, this would lead to transid mismatch if btrfs-restore chooses to
do device/chunk fixup.

This patch will fix such problem by pinning down all the log trees
blocks before fixing up the device and chunk trees.

Signed-off-by: Qu Wenruo <wqu@suse.com>