KVM: SVM: Skip OSVW variable updates if current CPU's errata are a subset

KVM: SVM: Skip OSVW variable updates if current CPU's errata are a subset

Elide the OSVW variable updates if the current CPU's set of errata are a
subset of the errata tracked in the global values, i.e. if no update is
needed.  There's no danger of under-reporting errata due to bailing early
as KVM is purely reducing the set of "known fixed" errata.  I.e. a racing
update on a different CPU with _more_ errata doesn't change anything if
the current CPU has the same or fewer errata relative to the status quo.

If another CPU is writing osvw_len, then "len" is guaranteed to be larger
than the new osvw_len and so the osvw_len update would be skipped anyways.

If another CPU is setting new bits in osvw_status, then "status" is
guaranteed to be a subset of the new osvw_status and the bitwise-OR would
be an effective nop anyways.

Link: https://patch.msgid.link/20251113231420.1695919-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>