KVM: x86/mmu: Add KVM_RMAP_MANY to replace open coded '1' and '1ul' literals

Replace all of the open coded '1' literals used to mark a PTE list as
having many/multiple entries with a proper define.  It's hard enough to
read the code with one magic bit, and a future patch to support "locking"
a single rmap will add another.

No functional change intended.

Link: https://lore.kernel.org/r/20240809194335.1726916-17-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>