[COMMON] mm: hpa: fix parameter that hides global identifier

[COMMON] mm: hpa: fix parameter that hides global identifier

The first parameter to is_movable_chunk() is start_pfn that is
declared as a global variable with the file scope. It is not harm
even though the parameter hids the global variable but it causes
the readers confusing so that it likely leads a mistake in the change
of some statements in the function.
Therefore the name of the first parameter to is_movable_chunk() is
changed and modified the logic of the body of is_movable_chunk() for
clear understanding.

Change-Id: I7b97aefb0b25dd09ca6c22fe0077b3d76b372210
Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>