kernel/svc: Sanitize addresses, permissions, and sizes within...

kernel/svc: Sanitize addresses, permissions, and sizes within svcMapSharedMemory() and svcUnmapSharedMemory()

Part of the checking done by the kernel is to check if the given
address and size are 4KB aligned, as well as checking if the size isn't
zero. It also only allows mapping shared memory as readable or
read/write, but nothing else, and so we shouldn't allow mapping as
anything else either.