mm: export symbol mmput_async

maillist inclusion
category: feature
bugzilla: 51855
CVE: NA

Reference: https://jpbrucker.net/git/linux/commit/?h=sva/2021-03-01&id=5207d639ca92f1e9aad02023fedaaafb3b91708d



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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.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Lijun Fang <fanglijun3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>