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/exofs/super.c b/fs/exofs/super.c
index d22cd168..6cafcad 100644
--- a/fs/exofs/super.c
+++ b/fs/exofs/super.c
@@ -754,6 +754,7 @@
 	sb->s_blocksize = EXOFS_BLKSIZE;
 	sb->s_blocksize_bits = EXOFS_BLKSHIFT;
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
+	sb->s_max_links = EXOFS_LINK_MAX;
 	atomic_set(&sbi->s_curr_pending, 0);
 	sb->s_bdev = NULL;
 	sb->s_dev = 0;