nilfs2: fix gcc warning at nilfs_checkpoint_is_mounted()

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: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/super.c

@@ -1020,7 +1020,7 @@ int nilfs_checkpoint_is_mounted(struct super_block *sb, __u64 cno)
 	struct dentry *dentry;
 	int ret;
 
-	if (cno < 0 || cno > nilfs->ns_cno)
+	if (cno > nilfs->ns_cno)
 		return false;
 
 	if (cno >= nilfs_last_cno(nilfs))