KVM: x86/mmu: Micro-optimize nEPT's bad memptype/XWR checks

Rework the handling of nEPT's bad memtype/XWR checks to micro-optimize
the checks as much as possible.  Move the check to a separate helper,
__is_bad_mt_xwr(), which allows the guest_rsvd_check usage in
paging_tmpl.h to omit the check entirely for paging32/64 (bad_mt_xwr is
always zero for non-nEPT) while retaining the bitwise-OR of the current
code for the shadow_zero_check in walk_shadow_page_get_mmio_spte().

Add a comment for the bitwise-OR usage in the mmio spte walk to avoid
future attempts to "fix" the code, which is what prompted this
optimization in the first place[*].

Opportunistically remove the superfluous '!= 0' and parantheses, and
use BIT_ULL() instead of open coding its equivalent.

The net effect is that code generation is largely unchanged for
walk_shadow_page_get_mmio_spte(), marginally better for
ept_prefetch_invalid_gpte(), and significantly improved for
paging32/64_prefetch_invalid_gpte().

Note, walk_shadow_page_get_mmio_spte() can't use a templated version of
the memtype/XRW as it works on the host's shadow PTEs, e.g. checks that
KVM hasn't borked its EPT tables.  Even if it could be templated, the
benefits of having a single implementation far outweight the few uops
that would be saved for NPT or non-TDP paging, e.g. most compilers
inline it all the way to up kvm_mmu_page_fault().

[*] https://lkml.kernel.org/r/20200108001859.25254-1-sean.j.christopherson@intel.com

Cc: Jim Mattson <jmattson@google.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>