vfs: check i_nlink limits in vfs_{mkdir,rename_dir,link}

New field of struct super_block - ->s_max_links.  Maximal allowed
value of ->i_nlink or 0; in the latter case all checks still need
to be done in ->link/->mkdir/->rename instances.  Note that this
limit applies both to directoris and to non-directories.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nilfs2/super.c

@@ -1059,6 +1059,7 @@ nilfs_fill_super(struct super_block *sb, void *data, int silent)
 	sb->s_export_op = &nilfs_export_ops;
 	sb->s_root = NULL;
 	sb->s_time_gran = 1;
+	sb->s_max_links = NILFS_LINK_MAX;
 
 	bdi = sb->s_bdev->bd_inode->i_mapping->backing_dev_info;
 	sb->s_bdi = bdi ? : &default_backing_dev_info;