page_alloc: clear nr_bootmem_regions in end_boot_allocator()

page_alloc: clear nr_bootmem_regions in end_boot_allocator()

... to make alloc_boot_pages() fail for late callers. Don't rely on
reaching the BOOT_BUG_ON(1) near the end of that function though, but
instead make this situation easier to distinguish from actual
allocation failures by adding an explicit check.

While there, make the iteration variable unsigned and guard against
underflow.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>