sbin/hammer: Fix terminology of buf_offset

sbin/hammer: Fix terminology of buf_offset

This commit just renames (local and struct field) variables.
No functional difference.

The way HAMMER userspace uses name "buf_offset" is misleading.
In kernel space, "buf_offset" is for arbitrary zone offsets that
are not limited to zone-2, however in userspace "buf_offset" is
used for zone-2. It should be renamed to "zone2_offset" so the
terminology being used in kernel and userspace are the same.

This is important because the name implies what's stored in
upper 4 bits of 64 bits offset, and having misleading variable
names tends to be error-prone (see the next commit).