mm: export symbol mm_access

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

Some devices can access process address spaces directly. When creating
such bond, to check that a process controlling the device is allowed to
access the target address space, the device driver uses mm_access(). Since
the drivers (in this case VFIO) can be built as a module, export the
mm_access symbol.

Cc: felix.kuehling@amd.com
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>