mm/memblock.c: warn if zero alignment was requested

mm/memblock.c: warn if zero alignment was requested

After updating all memblock users to explicitly specify SMP_CACHE_BYTES
alignment rather than use 0, it is still possible that uncovered users may
sneak in.  Add a WARN_ON_ONCE for such cases.

[[email protected]: use dump_stack() instead of WARN_ON_ONCE for the alignment checks]
Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: add apologetic comment]
Link: http://lkml.kernel.org/r/20181011060850.GA19822@rapoport-lnx
Signed-off-by: Mike Rapoport <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>