DMA, CMA: support alignment constraint on CMA region

DMA, CMA: support alignment constraint on CMA region

PPC KVM's CMA area management needs alignment constraint on CMA region.
So support it to prepare generalization of CMA area management
functionality.

Additionally, add some comments which tell us why alignment constraint
is needed on CMA region.

Signed-off-by: Joonsoo Kim <[email protected]>
Acked-by: Michal Nazarewicz <[email protected]>
Reviewed-by: Aneesh Kumar K.V <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Gleb Natapov <[email protected]>
Acked-by: Marek Szyprowski <[email protected]>
Tested-by: Marek Szyprowski <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Zhang Yanfei <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>