f2fs: give random value to i_generation

This follows to give random number to i_generation along with commit
23253068

 ("ext4: improve smp scalability for inode generation")

This can be used for DUN for UFS HW encryption.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>