sys/vfs/hammer: Add HAMMER_ROOT_VOLNO

sys/vfs/hammer: Add HAMMER_ROOT_VOLNO

The root volume# is always 0, so hammer userspace can use 0
instead of having a global variable RootVolNo that needs to
be set from -1 to 0 (and only 0).

Ondisk volume header even has fields whose name start with
vol0_xxx. This implies the root volume# is designed to be 0,
but nothing else.