btrfs: remove search_file_offset_in_bio

There is no need to search for a file offset in a bio, it is now always
provided in bbio->file_offset (set at bio allocation time since
0d495430 ("btrfs: set bbio->file_offset in alloc_new_bio")).  Just
use that with the offset into the bio.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>