x86/sev: Allow #VC exceptions on the VC2 stack

When code running on the VC2 stack causes a nested VC exception, the
handler will not handle it as expected but goes again into the error
path.

The result is that the panic() call happening when the VC exception
was raised in an invalid context is called recursively. Fix this by
checking the interrupted stack too and only call panic if it is not
the VC2 stack.

 [ bp: Fixup comment. ]

Fixes: 0786138c ("x86/sev-es: Add a Runtime #VC Exception Handler")
Reported-by: Xinyang Ge <xing@microsoft.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021080833.30875-3-joro@8bytes.org