Trigger KVM's various "unprotect gfn" paths if and only if the page fault
was a write to a write-protected gfn.  To do so, add a new page fault
return code, RET_PF_WRITE_PROTECTED, to explicitly and precisely track
such page faults.

If a page fault requires emulation for any MMIO (or any reason besides
write-protection), trying to unprotect the gfn is pointless and risks
putting the vCPU into an infinite loop.  E.g. KVM will put the vCPU into
an infinite loop if the vCPU manages to trigger MMIO on a page table walk.

Fixes: 14727754 ("kvm: svm: Add support for additional SVM NPF error codes")
Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20240831001538.336683-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>