s390/mm: Add mmap_assert_write_locked() check to crst_table_upgrade()

Add mmap_assert_write_locked() check to crst_table_upgrade() in order to
verify that no concurrent page table upgrades of an mm can happen. This
allows to remove the VM_BUG_ON() check which checks for the potential
inconsistent result of concurrent updates.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>