Use BUG_ON instead of a if condition followed by BUG.

Generated by: scripts/coccinelle/misc/bugon.cocci

Fixes: 4b526de5 ("KVM: x86: Check kvm_rebooting in kvm_spurious_fault()")
CC: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NJulia Lawall <julia.lawall@lip6.fr>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>