btrfs: rename btrfs_dio_private::logical_offset to file_offset

The naming of "logical_offset" can be confused with logical bytenr of
the dio range.

In fact it's file offset, and the naming "file_offset" is already widely
used in all other sites.

Just do the rename to avoid confusion.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>