KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array

There is no point in recalculating from scratch the total number of pages
in all memslots each time a memslot is created or deleted.  Use KVM's
cached nr_memslot_pages to compute the default max number of MMU pages.

Note that even with nr_memslot_pages capped at ULONG_MAX we can't safely
multiply it by KVM_PERMILLE_MMU_PAGES (20) since this operation can
possibly overflow an unsigned long variable.

Write this "* 20 / 1000" operation as "/ 50" instead to avoid such
overflow.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
[sean: use common KVM field and rework changelog accordingly]
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <d14c5a24535269606675437d5602b7dac4ad8c0e.1638817640.git.maciej.szmigiero@oracle.com>