sbin/hammer: Refactor left/right boundary elms in hammer show

sbin/hammer: Refactor left/right boundary elms in hammer show

hammer show doesn't need to pass around pointers to left/right
boundary elms in every function while walking the B-Tree.

Left/right elms are there to test whether a B-Tree node in
question fits in boundaries defined by its parent, which means
that hammer show always requires the right elm be (left+1), but
not some random elm after the left elm. Because of this, it
only needs to pass around the left elm.

The code is more efficient plus look better without bunch of
left/right stuff.