KVM: x86/mmu: Stop processing TDP MMU roots for test_age if young SPTE found

Return immediately if a young SPTE is found when testing, but not updating,
SPTEs.  The return value is a boolean, i.e. whether there is one young SPTE
or fifty is irrelevant (ignoring the fact that it's impossible for there to
be fifty SPTEs, as KVM has a hard limit on the number of valid TDP MMU
roots).

Link: https://lore.kernel.org/r/20241011021051.1557902-15-seanjc@google.com


[sean: use guard(rcu)(), as suggested by Paolo]
Signed-off-by: Sean Christopherson <seanjc@google.com>