KVM: guest_memfd: Drop a superfluous local var in kvm_gmem_fault_user_mapping()

KVM: guest_memfd: Drop a superfluous local var in kvm_gmem_fault_user_mapping()

Drop the local "int err" that's buried in the middle guest_memfd's user
fault handler to avoid the potential for variable shadowing, e.g. if an
"err" variable were also declared at function scope.

No functional change intended.

Link: https://lore.kernel.org/r/20251007222733.349460-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>