radix_tree: for lookup_le find low bits better

[dougm]

Summary

At one point, lookup_le needs to know if any of the first 'slot' bits in popmap are nonzero. Since we know that popmap is nonzero, this change reforms that test in a way that saves one byte on amd64.

As it happens, saving one byte in vm_radix_lookup_le means saving 15 nop bytes as well, so that the function shrinks by 16 bytes.

Test Plan

Tested by building a kernel.

[dougm]

Shrink code size by 2 more bytes.

[dougm]

Update to leave out vm_radix.c.