mm/cma.c: switch to bitmap_zalloc() for cma bitmap allocation

mm/cma.c: switch to bitmap_zalloc() for cma bitmap allocation

kzalloc() is used for cma bitmap allocation in cma_activate_area(),
switch to bitmap_zalloc() for clarity.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yunfeng Ye <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Yue Hu <[email protected]>
Cc: Peng Fan <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Ryohei Suzuki <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Doug Berger <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>