sbin/hammer: Fix conditional for Hammer_FSId initialization

sbin/hammer: Fix conditional for Hammer_FSId initialization

Hammer_FSId is initialized when no entry in the VolList yet,
but this becomes an issue when a new volume (that hasn't joined
the HAMMER filesystem yet) is the first created volume_info.

If the second volume_info is for the existing HAMMER volume,
it is likely that
bcmp(&Hammer_FSId, &ondisk->vol_fsid, sizeof(Hammer_FSId))
does not match and results in the following error.
"load_volume: %s: FSId does match other volumes!"

The only reason this hasn't become an issue so far is because
no existing hammer command creates volume_info in that order.