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/jfs/super.c b/fs/jfs/super.c
index 682bca6..4661ad7 100644
--- a/fs/jfs/super.c
+++ b/fs/jfs/super.c
@@ -441,6 +441,7 @@
 		return -ENOMEM;
 
 	sb->s_fs_info = sbi;
+	sb->s_max_links = JFS_LINK_MAX;
 	sbi->sb = sb;
 	sbi->uid = sbi->gid = sbi->umask = -1;