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.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: fix used-unintialised warning]
Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>