x86/sev-es: Handle string port IO to kernel memory properly

Don't assume dest/source buffers are userspace addresses when manually
copying data for string I/O or MOVS MMIO, as {get,put}_user() will fail
if handed a kernel address and ultimately lead to a kernel panic.

When invoking INSB/OUTSB instructions in kernel space in a
SEV-ES-enabled VM, the kernel crashes with the following message:

  "SEV-ES: Unsupported exception in #VC instruction emulation - can't continue"

Handle that case properly.

 [ bp: Massage commit message. ]

Fixes: f980f9c3

 ("x86/sev-es: Compile early handler code into kernel image")
Signed-off-by: Hyunwook (Wooky) Baek <baekhw@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: David Rientjes <rientjes@google.com>
Link: https://lkml.kernel.org/r/20210110071102.2576186-1-baekhw@google.com