btrfs: scrub: fix a wrong error type when metadata bytenr mismatches

When the bytenr doesn't match for a metadata tree block, we will report
it as an csum error, which is incorrect and should be reported as a
metadata error instead.

Fixes: a3ddbaeb ("btrfs: scrub: introduce a helper to verify one metadata block")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>