mm/cma: fix cma bitmap aligned mask computing

The current cma bitmap aligned mask computation is incorrect. It could
cause an unexpected alignment when using cma_alloc() if the wanted align
order is larger than cma->order_per_bit.

Take kvm for example (PAGE_SHIFT = 12), kvm_cma->order_per_bit is set to
6. When kvm_alloc_rma() tries to alloc kvm_rma_pages, it will use 15 as
the expected align value. After using the current implementation however,
we get 0 as cma bitmap aligned mask other than 511.

This patch fixes the cma bitmap aligned mask calculation.

[[e-mail]: coding-style fixes]
Signed-off-by: Weijie Yang <[e-mail]>
Acked-by: Michal Nazarewicz <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Cc: "Aneesh Kumar K.V" <[e-mail]>
Cc: <[e-mail]> [3.17]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/cma.c

···
 
 static unsigned long cma_bitmap_aligned_mask(struct cma *cma, int align_order)
 {
-	return (1UL << (align_order >> cma->order_per_bit)) - 1;
+	if (align_order <= cma->order_per_bit)
+		return 0;
+	return (1UL << (align_order - cma->order_per_bit)) - 1;
 }
 
 static unsigned long cma_bitmap_maxno(struct cma *cma)