btrfs: Don't opencode btrfs_find_name_in_backref in backref_in_log

Direct replacement, though note that the inside of the loop in
btrfs_find_name_in_backref is organized in a slightly different way but
is equvalent.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>