Fix the following build warning:

 fs/nilfs2/super.c: In function 'nilfs_checkpoint_is_mounted':
 fs/nilfs2/super.c:1023:10: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   if (cno < 0 || cno > nilfs->ns_cno)
           ^

This warning indicates that the comparision "cno < 0" is useless because
variable "cno" has an unsigned integer type "__u64".
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Reported-by: NDavid Binderman <dcb314@hotmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>