usr.sbin/makefs/hammer2: Use blkno -1 for logical vnode for clarity

usr.sbin/makefs/hammer2: Use blkno -1 for logical vnode for clarity

At logical vnode layer, only bio_offset from uio_offset is used,
in the same way DragonFly VFS does. Since blkno should never be used,
set it to -1.

Then the bio_offset turns into bref's data_off through chain lookup.
At physical vnode layer, bref's data_off turns into a buf associated
with dio's devvp.

At this point dio's devvp buf needs a valid blkno (but not a valid
offset as getblk/bread/bwrite functions in ffs/buf.c are based on
FreeBSD VFS) to calculate file offset within image.