Use better bitmap_zalloc() to allocate bitmap.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Link: https://lore.kernel.org/r/20210529111510.186355-1-wangkefeng.wang@huawei.comSigned-off-by: Will Deacon <will@kernel.org>