KVM: x86: Use reserved_gpa_bits to calculate reserved PxE bits

Use reserved_gpa_bits, which accounts for exceptions to the maxphyaddr
rule, e.g. SEV's C-bit, for the page {table,directory,etc...} entry (PxE)
reserved bits checks.  For SEV, the C-bit is ignored by hardware when
walking pages tables, e.g. the APM states:

  Note that while the guest may choose to set the C-bit explicitly on
  instruction pages and page table addresses, the value of this bit is a
  don't-care in such situations as hardware always performs these as
  private accesses.

Such behavior is expected to hold true for other features that repurpose
GPA bits, e.g. KVM could theoretically emulate SME or MKTME, which both
allow non-zero repurposed bits in the page tables.  Conceptually, KVM
should apply reserved GPA checks universally, and any features that do
not adhere to the basic rule should be explicitly handled, i.e. if a GPA
bit is repurposed but not allowed in page tables for whatever reason.

Refactor __reset_rsvds_bits_mask() to take the pre-generated reserved
bits mask, and opportunistically clean up its code, e.g. to align lines
and comments.

Practically speaking, this is change is a likely a glorified nop given
the current KVM code base.  SEV's C-bit is the only repurposed GPA bit,
and KVM doesn't support shadowing encrypted page tables (which is
theoretically possible via SEV debug APIs).

Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>