Optimize vm_radix_lookup_ge() and vm_radix_lookup_le().  Specifically,
change the way that these functions ascend the tree when the search for a
matching leaf fails at an interior node.  Rather than returning to the root
of the tree and repeating the lookup with an updated key, maintain a stack
of interior nodes that were visited during the descent and use that stack
to resume the lookup at the closest ancestor that might have a matching
descendant.

Sponsored by:	EMC / Isilon Storage Division
Reviewed by:	attilio
Tested by:	pho