KVM: x86: Widen x86_exception's error_code to 64 bits

KVM: x86: Widen x86_exception's error_code to 64 bits

Widen the error_code field in struct x86_exception from u16 to u64 to
accommodate AMD's NPF error code, which defines information bits above
bit 31, e.g. PFERR_GUEST_FINAL_MASK (bit 32), and PFERR_GUEST_PAGE_MASK
(bit 33).

Retain the u16 type for the local errcode variable in walk_addr_generic
as the walker synthesizes conventional #PF error codes that are
architecturally limited to bits 15:0.

Signed-off-by: Kevin Cheng <chengkev@google.com>
Link: https://patch.msgid.link/20260522232701.3671446-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>