f2fs: prevent possible int overflow in dir_block_index()

The result of multiplication between values derived from functions
dir_buckets() and bucket_blocks() *could* technically reach
2^30 * 2^2 = 2^32.

While unlikely to happen, it is prudent to ensure that it will not
lead to integer overflow. Thus, use mul_u32_u32() as it's more
appropriate to mitigate the issue.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 38431545 ("f2fs: introduce large directory support")
Cc: stable@vger.kernel.org
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>