mm/page_alloc.c: simplify calculation of combined index of adjacent buddy lists

mm/page_alloc.c: simplify calculation of combined index of adjacent buddy lists

The previous approach of calucation of combined index was

	page_idx & ~(1 << order))

but we have same result with

	page_idx & buddy_idx

This reduces instructions slightly as well as enhances readability.

[[email protected]: coding-style fixes]
[[email protected]: fix used-unintialised warning]
Signed-off-by: KyongHo Cho <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>