CMA: fix CONFIG_CMA_SIZE_MBYTES overflow in 64bit

In 64bit system, if you set CONFIG_CMA_SIZE_MBYTES>=2048, it will
overflow and size_bytes will be a big wrong number.

Set CONFIG_CMA_SIZE_MBYTES=2048 and you will get an info below
during system boot:

*********
cma: Failed to reserve 17592186042368 MiB
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Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>