mm: export symbol find_get_task_by_vpid

maillist inclusion
category: feature
bugzilla: 51855
CVE: NA

Reference: https://jpbrucker.net/git/linux/commit/?h=sva/2021-03-01&id=176b3454e53ffdd4a4f6dbc51f128f1f35a1b357



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Userspace drivers implemented with VFIO might want to bind sub-processes
to their devices. In a VFIO ioctl, they provide a pid that is used to find
a task and its mm. Since VFIO can be built as a module, export the
find_get_task_by_vpid 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>