btrfs: add 64bit safe helper for power of two checks

As is_power_of_two takes unsigned long, it's not safe on 32bit
architectures, but we could pass any u64 value in seveal places. Add a
separate helper and also an alias that better expresses the purpose for
which the helper is used.

Signed-off-by: David Sterba <dsterba@suse.com>