btrfs-progs: fsck-tests: Introduce test case with keyed data backref with the extent...

btrfs-progs: fsck-tests: Introduce test case with keyed data backref with the extent offset

Add the testcase for false alert of data extent backref lost with the
extent offset.

The image can be reproduced by the following commands:
------
dev=~/test.img
mnt=/mnt/btrfs

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

mkfs.btrfs $dev
mount $dev $mnt

for i in `seq 1 10`; do
	xfs_io -f -c "pwrite 0 2K" $mnt/file$i
done

xfs_io -f -c "falloc 0 64K" $mnt/file11

for i in `seq 1 32`; do
	xfs_io -f -c "reflink $mnt/file11 0 $(($i * 64))K 64K" $mnt/file11
done

xfs_io -f -c "reflink $mnt/file11 32K $((33 * 64))K 32K" $mnt/file11

btrfs subvolume snapshot $mnt $mnt/snap1

umount $mnt
btrfs-image -c9 $dev extent_data_ref.img
------

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>