Dedup the code for generating (most of) the per-type PT_* masks in
paging_tmpl.h.  The relevant macros only vary based on the number of bits
per level, and that smidge of info is already provided in a common form
as PT_LEVEL_BITS.

No functional change intended.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20220614233328.3896033-7-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>