s390/pkey: Prevent overflow in size calculation for memdup_user()

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

s390/pkey: Prevent overflow in size calculation for memdup_user()

Number of apqn target list entries contained in 'nr_apqns' variable is determined by userspace via an ioctl call so the result of the product in calculation of size passed to memdup_user() may overflow.

In this case the actual size of the allocated area and the value describing it won't be in sync leading to various types of unpredictable behaviour later.

Use a proper memdup_array_user() helper which returns an error if an overflow is detected. Note that it is different from when nr_apqns is initially zero - that case is considered valid and should be handled in subsequent pkey_handler implementations.

Found by Linux Verification Center (linuxtesting.org).