btrfs-progs: check/lowmem: Fix false alert of data extent backref lost for snapshot

btrfs-progs: check/lowmem: Fix false alert of data extent backref lost for snapshot

Btrfs lowmem check reports the following false alert:
------
ERROR: file extent[267 2162688] root 256 owner 5 backref lost
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The file extent is in the leaf which is shared by file tree 256 and fs
tree.
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leaf 30605312 items 46 free space 4353 generation 7 owner 5
......
        item 45 key (267 EXTENT_DATA 2162688) itemoff 5503 itemsize 53
                generation 7 type 2 (prealloc)
                prealloc data disk byte 13631488 nr 65536
                prealloc data offset 32768 nr 32768
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And there is the corresponding extent_data_ref item in the extent tree.
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        item 1 key (13631488 EXTENT_DATA_REF 1007496934287921081) itemoff 15274 itemsize 28
                extent data backref root 5 objectid 267 offset 2129920 count 1
------

The offset of EXTENT_DATA_REF which is the hash of the owner root objectid,
the inode number and the calculated offset (file offset - extent offset).

What caused the false alert is the code mix up the owner root objectid and
the file tree objectid.

Fixes: b0d360b541f0 ("btrfs-progs: check: introduce function to check data backref in extent tree")
Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>