Try to locate any instances where pmap_enter*() is called with a kernel

Try to locate any instances where pmap_enter*() is called with a kernel
virtual address on a pmap other then kernel_pmap by adding some debugging
printfs.

Remove pmap_kernel() (which just returned a pointer to kernel_pmap), and
change the kernel_pmap global from a pointer to a directly accessed
structure.  Adjust code accordingly.