sbin/hammer: Fix HammerVersion initialization added by 4c09d9c4

sbin/hammer: Fix HammerVersion initialization added by 4c09d9c4

4c09d9c4 added HammerVersion initialization code to load_volume(),
but this must be done only if volume is a root volume.

hammer version-upgrade only overwrites vol_version field of root
volume header, thus load_volume() updating HammerVersion using
non root volume(s) after once initialized using the root volume
breaks everything that volume version involves.

If HammerVersion isn't initialized by reading the volume header
(i.e. initialzied via ioctl when fs is mounted), it doesn't break
anything at least in the same way as this.