Revert part of the previous uvm_km_zalloc(9) to km_alloc(9) commit.

Revert part of the previous uvm_km_zalloc(9) to km_alloc(9) commit.
The chunk with mapping for pm_pdir_intel causes crashes for sthen@
and bluhm@.  i386 pagedaemon panic: kernel diagnostic assertion
"pg->wire_count == 0" failed: file "/usr/src/sys/uvm/uvm_page.c",
line 1265
One of my 8 CPU 3 GB RAM machines can reproduce it when building
clang with make -j 9.  This commit hides the underlying bug.
OK mpi@