nilfs2: eliminate the shared counter and spinlock for i_generation

nilfs2: eliminate the shared counter and spinlock for i_generation

Use get_random_u32() as the source for inode->i_generation for new inodes,
and eliminate the original source, the shared counter ns_next_generation
along with its exclusive access spinlock ns_next_gen_lock.

Link: https://lkml.kernel.org/r/20240826174116.5008-5-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>