arm: mm: Exclude additional mem_map entries from free

A previous patch addressed the issue of move_freepages_block()
trampling on erronously freed mem_map entries for the bank end
pfn. We also need to restrict the start pfn in a
complementary manner.

Also make macro usage consistent by adopting the use of
round_down and round_up.

Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
(cherry picked from commit ccd78a45dcd1d8255edddcf9062e0b0d34d8d27f)