f2fs: give random value to i_generation

This follows to give random number to i_generation along with commit
2325306802 ("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>

fs/f2fs/f2fs.h

@@ -1242,8 +1242,6 @@ struct f2fs_sb_info {
 
 	unsigned int nquota_files;		/* # of quota sysfile */
 
-	u32 s_next_generation;			/* for NFS support */
-
 	/* # of pages, see count_type */
 	atomic_t nr_pages[NR_COUNT_TYPE];
 	/* # of allocated blocks */