radix_tree: compute slot from keybarr

The computation of keybarr(), the function that determines when a
search has failed at a non-leaf node, can be done in a way that
computes the 'slot' value when keybarr() fails, which is exactly when
slot() would next be invoked. Computing things this way saves space in
search loops.

This reduces the amd64 coding of the search loop in vm_radix_lookup
from 40 bytes to 28 bytes.

Reviewed by: alc
Tested by: pho (as part of a larger change)
Differential Revision: https://reviews.freebsd.org/D41235