mm/memblock.c: hardcode the end_pfn being -1

mm/memblock.c: hardcode the end_pfn being -1

This is just a cleanup.  It aids handling the special end case in the
next commit.

[[email protected]: make it work against current -linus, not against -mm]
[[email protected]: make it work against current -linus, not against -mm some more]
Link: http://lkml.kernel.org/r/1ca478d4269125a99bcfb1ca04d7b88ac1aee924.1520011944.git.neelx@redhat.com
Signed-off-by: Daniel Vacek <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>