mmu: make flush_tlb_all() migration-safe

If a thread which invokes flush_tlb_all() is migrated between the
call to flush_tlb_local() and send_allbutself(), the CPU onto which it
was migrated would not get its TLB flushed.

Spotted during code inspection.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>