KVM: SVM: Invert user pointer casting in SEV {en,de}crypt helpers

Invert the user pointer params for SEV's helpers for encrypting and
decrypting guest memory so that they take a pointer and cast to an
unsigned long as necessary, as opposed to doing the opposite.  Tagging a
non-pointer as __user is confusing and weird since a cast of some form
needs to occur to actually access the user data.  This also fixes Sparse
warnings triggered by directly consuming the unsigned longs, which are
"noderef" due to the __user tag.

Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210506231542.2331138-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>