mm: Change tmpfs size base on ddr size.

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4JMLR


CVE: NA
-------------------

If it enable COHERENT_DEVICE and config HBM for device,
the tmpfs will statistics all memory including HBM, which
couldn't be used by system. When the system runs out of memory,
statistical residual memory but also left a lot,
so this will cause many problems.

Signed-off-by: Lijun Fang <fanglijun3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>