KVM: x86: Rename and invert async #PF's send_user_only flag to send_always

KVM: x86: Rename and invert async #PF's send_user_only flag to send_always

Rename send_user_only to avoid "user", because KVM's ABI is to not inject
page faults into CPL0, whereas "user" in x86 is specifically CPL3.  Invert
the polarity to keep the naming simple and unambiguous.  E.g. while KVM
often refers to CPL0 as "kernel", that terminology isn't ubiquitous, and
"send_kernel" could be misconstrued as "send only to kernel".

Link: https://lore.kernel.org/r/20250215010609.1199982-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>