btrfs-progs: convert: Fix a bug which fails to insert hole file extent

btrfs-progs: convert: Fix a bug which fails to insert hole file extent

When copying inode, if there is a file referring part of a hole range,
convert will fail.

The problem is, when calculating real extent bytenr, it doesn't check if
the original extent is a hole.

In case the orinal extent is a hole, we still calculate bytenr using
file_pos - found_extent_file_pos, causing non-zero value, and later
btrfs_record_file_extent() detects that we are pointing to non-exist
extent and aborts convert.

Fix it by checking the disk_bytenr before calculating real disk bytenr.

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