KVM: s390/mm: Fix page table locking vs. split pmd lock

KVM: s390/mm: Fix page table locking vs. split pmd lock

commit 4dd74f8dad5cb6856db6cf8d08d7581e56b20b8b (s390/mm: enable
split page table lock for PMD level) activated the split pmd lock
for s390. Turns out that we missed one place: We also have to take
the pmd lock instead of the page table lock when we reallocate the
page tables (==> changing entries in the PMD) during sie enablement.

Cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>