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 <[email protected]>
Reported-by: David Binderman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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))