mm: migrate: initialize err in do_migrate_pages

After commit 236c32eb ("mm: migrate: clean up migrate_prep{_local}")',
do_migrate_pages can return uninitialized variable 'err' (which is
propagated to user-space as error) when 'from' and 'to' nodesets are
identical.  This can be reproduced with LTP migrate_pages01, which calls
migrate_pages() with same set for both old/new_nodes.

Add 'err' initialization back.

Link: https://lkml.kernel.org/r/456a021c7ef3636d7668cec9dcb4a446a4244812.1609855564.git.jstancek@redhat.com
Fixes: 236c32eb ("mm: migrate: clean up migrate_prep{_local}")
Signed-off-by: NJan Stancek <jstancek@redhat.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Acked-by: NYang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Song Liu <songliubraving@fb.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1111,7 +1111,7 @@ int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
 		     const nodemask_t *to, int flags)
 {
 	int busy = 0;
-	int err;
+	int err = 0;
 	nodemask_t tmp;
 
 	migrate_prep();