KVM: SVM: Guard against DEACTIVATE when performing WBINVD/DF_FLUSH

The SEV firmware DEACTIVATE command disassociates an SEV guest from an
ASID, clears the WBINVD indicator on all threads and indicates that the
SEV firmware DF_FLUSH command must be issued before the ASID can be
re-used. The SEV firmware DF_FLUSH command will return an error if a
WBINVD has not been performed on every thread before it has been invoked.
A window exists between the WBINVD and the invocation of the DF_FLUSH
command where an SEV firmware DEACTIVATE command could be invoked on
another thread, clearing the WBINVD indicator. This will cause the
subsequent SEV firmware DF_FLUSH command to fail which, in turn, results
in the SEV firmware ACTIVATE command failing for the reclaimed ASID.
This results in the SEV guest failing to start.

Use a mutex to close the WBINVD/DF_FLUSH window by obtaining the mutex
before the DEACTIVATE and releasing it after the DF_FLUSH. This ensures
that any DEACTIVATE cannot run before a DF_FLUSH has completed.

Fixes: 59414c989220 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_START command")
Tested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>