KVM: s390: Fix potential races in dat skey functions

In the Linux kernel, the following vulnerability has been resolved:

KVM: s390: Fix potential races in dat skey functions

When dat_cond_set_storage_key() finds a large page, it will
conditionally set the storage key in absolute memory using
large_crste_to_phys() to get the absolute address.

There is a race window between dat_entry_walk() and
large_crste_to_phys(): the large page could have been split
concurrently, and large_crste_to_phys() might be called with a crste
that does not designate a large page, leading to crashes.

Similar issues were also present in dat_set_storage_key().

dat_get_storage_key() and dat_reset_reference_bit() did instead check
for a potential concurrent splitting of the large page, but then
handled it incorrectly.

Fix by performing a READ_ONCE on the crste pointer, checking and using
the result, instead of dereferencing the pointer again. In case a race
is detacted, try dat_entry_walk() again.