f2fs: fix error return code in f2fs_fill_super()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.
Introduce by commit c0d39e(f2fs: fix return values from validate superblock)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>