mm: export symbol mmput_async

hulk inclusion
category: feature
bugzilla: 14369
CVE: NA
-------------------

In some cases releasing a mm bound to a device might invoke an exit
handler, that takes a lock already held by the function calling mmput().
This is the case for VFIO, which needs to call mmput_async to avoid a
deadlock. Other drivers using SVA might follow. Since they can be built as
modules, export the mmput_async symbol.

Cc: akpm@linux-foundation.org
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Fang Lijun <fanglijun3@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>