arm64: KVM: Tighten guest core register access from userspace

We currently allow userspace to access the core register file
in about any possible way, including straddling multiple
registers and doing unaligned accesses.

This is not the expected use of the ABI, and nobody is actually
using it that way. Let's tighten it by explicitly checking
the size and alignment for each field of the register file.

Cc: <stable@vger.kernel.org>
Fixes: 2f4a07c5 ("arm64: KVM: guest one-reg interface")
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[maz: rewrote Dave's initial patch to be more easily backported]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>