f2fs: fix double free of unicode map

In case of retrying fill_super with skip_recovery,
s_encoding for casefold would not be loaded again even though it's
already been freed because it's not NULL.
Set NULL after free to prevent double freeing when unmount.

Fixes: eca4873e ("f2fs: Use generic casefolding support")
Signed-off-by: NHyeongseok Kim <hyeongseok@gmail.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -3918,6 +3918,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 
 #ifdef CONFIG_UNICODE
 	utf8_unload(sb->s_encoding);
+	sb->s_encoding = NULL;
 #endif
 free_options:
 #ifdef CONFIG_QUOTA