x86/svm: Don't unconditionally use a new ASID in svm_invlpg_intercept()

x86/svm: Don't unconditionally use a new ASID in svm_invlpg_intercept()

paging_invlpg() already returns a boolean indicating whether an invalidation
is necessary or not.  A return value of 0 indicates that the specified virtual
address wasn't shadowed (or has already been flushed), cannot currently be
cached in the TLB.

This is a performance optimisation.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>