fs: Fix theoretical division by 0 in super_cache_scan().

commit 475d0db7 upstream.

total_objects could be 0 and is used as a denom.

While total_objects is a "long", total_objects == 0 unlikely happens for
3.12 and later kernels because 32-bit architectures would not be able to
hold (1 << 32) objects. However, total_objects == 0 may happen for kernels
between 3.1 and 3.11 because total_objects in prune_super() was an "int"
and (e.g.) x86_64 architecture might be able to hold (1 << 32) objects.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/super.c

@@ -68,6 +68,8 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 
 	total_objects = sb->s_nr_dentry_unused +
 			sb->s_nr_inodes_unused + fs_objects + 1;
+	if (!total_objects)
+		total_objects = 1;
 
 	if (sc->nr_to_scan) {
 		int	dentries;