btrfs: relocation: return bool from btrfs_should_ignore_reloc_root

btrfs_should_ignore_reloc_root() is a predicate so it should return
bool.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>