memblock: align freed memory map on pageblock boundaries with SPARSEMEM

When CONFIG_SPARSEMEM=y the ranges of the memory map that are freed are not
aligned to the pageblock boundaries which breaks assumptions about
homogeneity of the memory map throughout core mm code.

Make sure that the freed memory map is always aligned on pageblock
boundaries regardless of the memory model selection.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Tony Lindgren <tony@atomide.com>