KVM: SVM: Move lock-protected allocation of SEV ASID into a separate helper

KVM: SVM: Move lock-protected allocation of SEV ASID into a separate helper

Extract the lock-protected parts of SEV ASID allocation into a new helper
and opportunistically convert it to use guard() when acquiring the mutex.

Preserve the goto even though it's a little odd, as it's there's a fair
amount of subtlety that makes it surprisingly difficult to replicate the
functionality with a loop construct, and arguably using goto yields the
most readable code.

No functional change intended.

Signed-off-by: Carlos López <clopez@suse.de>
[sean: move code to separate helper, rework shortlog+changelog]
Link: https://patch.msgid.link/20260310234829.2608037-21-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>