um: Optimize Flush TLB for force/fork case

When UML handles a fork the page tables need to be brought up
to date. That was done using brute force - full tlb flush.

This is actually unnecessary, because the mapped-in mappings are
all correct and the only mappings which need to be updated
after a flush are any unmaps (so that paging works) as well as
any pending protection changes.

This optimization squeezes out up to 3% from a full kernel rebuild
time under memory pressure.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>