f2fs: Fix a return value in case of error in 'f2fs_fill_super'

err must be set to -ENOMEM, otherwise we return 0.

Fixes: a912b54d

 ("f2fs: split bio cache")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>