pmap_protect(), and pmap_copy() have optimizations for regions
larger than PMAP_TSB_THRESH (which works out to 16MB). This
caused a panic in tsb_foreach for kernel mappings, since
pm->pm_tsb is NULL in that case. This fix teaches tsb_foreach
to use the kernel's tsb in that case.
Submitted by: Michael Plass
MFC after: 3 days