btrfs-progs: fsck-test: Add new image with shared block ref only metadata backref

btrfs-progs: fsck-test: Add new image with shared block ref only metadata backref

The image is dumped by modifying kernel to sleep long enough before
merging relocation trees, so we can just copy the whole image to other
place before kernel begins to merge reloc trees.

And the base image is created by the following script to bump metadata
size:
------
dev=~/test.img
mnt=/mnt/btrfs

umount $mnt &> /dev/null
fallocate -l 128M $dev

mkfs.btrfs -f -n 4k -m single -d single $dev
mount $dev $mnt -o nospace_cache,max_inline=2048

btrfs subvolume create $mnt/src
for i in $(seq -w 0 128); do
	xfs_io -f -c "pwrite 0 2k" $mnt/src/file_$i > /dev/null
done
for i in $(seq -w 0 64); do
	btrfs subvolume snapshot $mnt/src/ $mnt/snapshot_$i
	touch $mnt/snapshot_$i/new
done
sync
------

The image triggers several corner cases that the old lowmem mode didn't
consider.

Like metadata backref with FULL_BACKREF flag and only SHARED_BLOCK_REF
backrefs for metadata.  And several tree reloc trees with shared
leaves/nodes to confuse old lowmem mode.

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