ANDROID: mm/memory_hotplug: Don't special case memory_block_size_bytes

If add_memory_subsection() is called with a size of
memory_block_size_bytes, it calls into add_memory(), which declares
the region as system ram, and adds it to the buddy allocator. This
is inconsistent with the behavior of add_memory_subsection() for
other sizes, for which it does not add the memory to buddy and
instead reserves it for the caller's private use.

Bug: 210008865
Fixes: 417ac617 ("ANDROID: mm/memory_hotplug: implement {add/remove}_memory_subsection")
Change-Id: Iefb69b0b4e96af670d0e65c325a9538d14b460e3
Signed-off-by: Patrick Daly <quic_pdaly@quicinc.com>