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>
diff --git a/fs/ufs/super.c b/fs/ufs/super.c
index 5246ee3..ec25d09 100644
--- a/fs/ufs/super.c
+++ b/fs/ufs/super.c
@@ -1157,6 +1157,7 @@
 			    "fast symlink size (%u)\n", uspi->s_maxsymlinklen);
 		uspi->s_maxsymlinklen = maxsymlen;
 	}
+	sb->s_max_links = UFS_LINK_MAX;
 
 	inode = ufs_iget(sb, UFS_ROOTINO);
 	if (IS_ERR(inode)) {