KVM: x86: Drop superfluous caching of KVM_ASYNC_PF_SEND_ALWAYS

KVM: x86: Drop superfluous caching of KVM_ASYNC_PF_SEND_ALWAYS

Drop kvm_vcpu_arch.apf.send_always and instead use msr_en_val as the source
of truth to reduce the probability of operating on stale data.  This fixes
flaws where KVM fails to update send_always when APF is explicitly
disabled by the guest or implicitly disabled by KVM on INIT.  Absent other
bugs, the flaws are benign as KVM *shouldn't* consume send_always when PV
APF support is disabled.

Simply delete the field, as there's zero benefit to maintaining a separate
"cache" of the state.

Opportunistically turn the enabled vs. disabled logic at the end of
kvm_pv_enable_async_pf() into an if-else instead of using an early return,
e.g. so that it's more obvious that both paths are "success" paths.

Fixes: 6adba5274206 ("KVM: Let host know whether the guest can handle async PF in non-userspace context.")
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20260406225359.1245490-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>