mm/mempolicy: remove or narrow the lock on current

It is not necessary to hold the lock of current when setting nodemask of
a new policy.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200921040416.86185-1-richard.weiyang@linux.alibaba.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>