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/logfs/super.c b/fs/logfs/super.c
index c9ee7f5..b1a491a 100644
--- a/fs/logfs/super.c
+++ b/fs/logfs/super.c
@@ -542,6 +542,7 @@
 	 * the filesystem incompatible with 32bit systems.
 	 */
 	sb->s_maxbytes	= (1ull << 43) - 1;
+	sb->s_max_links = LOGFS_LINK_MAX;
 	sb->s_op	= &logfs_super_operations;
 	sb->s_flags	= flags | MS_NOATIME;