btrfs: change return type to bool type of check_eb_alignment()

The check function pattern is supposed to return true/false, currently
there's only one error code.

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