The tlb flushes in pmap_set_{l1,l2,l3} use ranges that don't fully make sense.

The tlb flushes in pmap_set_{l1,l2,l3} use ranges that don't fully make sense.
But those tlb flushes shouldn't be necessary anyway, so simply remove them.
Simplify the tlb flushing code now that we no longer flush ranges, and
revive the branch that doesn't flush a specific ASID for the kernel pmap
since its mappings are global.

ok patrick@, visa@