s390/mm: fix incorrect ASCE after crst_table_downgrade

The switch_mm function does nothing in case the prev and next mm
are the same. It can happen that a crst_table_downgrade has changed
the top-level pgd in the meantime on a different CPU. Always store
the new ASCE to be picked up in entry.S.

[heiko.carstens@de.ibm.com]: Bug was introduced with git commit
53e857f3 ("s390/mm,tlb: race of lazy TLB flush vs. recreation
of TLB entries") and causes random crashes due to broken page tables
being used.

Reported-by: Dominik Vogt <vogt@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>