mm/mempolicy.c: avoid use uninitialized preferred_node

mm/mempolicy.c: avoid use uninitialized preferred_node

Alexander reported a use of uninitialized memory in __mpol_equal(),
which is caused by incorrect use of preferred_node.

When mempolicy in mode MPOL_PREFERRED with flags MPOL_F_LOCAL, it uses
numa_node_id() instead of preferred_node, however, __mpol_equal() uses
preferred_node without checking whether it is MPOL_F_LOCAL or not.

[[email protected]: slight comment tweak]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: fc36b8d3d819 ("mempolicy: use MPOL_F_LOCAL to Indicate Preferred Local Policy")
Signed-off-by: Yisheng Xie <[email protected]>
Reported-by: Alexander Potapenko <[email protected]>
Tested-by: Alexander Potapenko <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Dmitriy Vyukov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>