btrfs-progs: lowmem check: Fix false backref lost warning for keyed extent data ref

btrfs-progs: lowmem check: Fix false backref lost warning for keyed extent data ref

For keyed extent ref, its offset is calculated offset (file offset -
file extent offset), just like inlined extent data ref.

However the code is using file offset to hash extent data ref offset,
causing false backref lost warning like:
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ERROR: data extent[16913485824 7577600] backref lost
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Fixes: b0d360b541f0 ("btrfs-progs: check: introduce function to check data backref in extent tree")
Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>