btrfs: relocation: return bool from btrfs_should_ignore_reloc_root

[ Upstream commit 32f2abca ]

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>
Stable-dep-of: db7e68b5 ("btrfs: drop the backref cache during relocation if we commit")
Signed-off-by: Sasha Levin <sashal@kernel.org>