nfs_instantiate(): prevent multiple aliases for directory inode

Since NFS allows open-by-fhandle, we have to cope with the possibility
of mkdir vs. open-by-guessed-handle races. A local filesystem could
decide what the inumber of the new object will be and insert a locked
inode with that inumber into icache _before_ the on-disk data structures
begin to look good and unlock it only once it has a dentry alias, so
that open-by-handle coming first would quietly fail and mkdir coming
first would have open-by-handle grab its dentry.

For NFS it's a non-starter - the icache key is server-supplied fhandle
and we do not get that until the object has been fully created on server.
We really have to deal with the possibility that open-by-handle gets
the in-core inode and attaches a dentry to it before mkdir does.

Solution: let nfs_mkdir() use d_splice_alias() to catch those. We can
* get an error. Just return it to our caller.
* get NULL - no preexisting dentry aliases, we'd just done what
d_add() would've done. Success.
* get a reference to preexisting alias. In that case the alias
had been moved in place of nfs_mkdir() argument (and hashed there), while
nfs_mkdir() argument is left unhashed negative. Which is just fine for
->mkdir() callers, all we need is to release the reference we'd got from
d_splice_alias() and report success.

Cc: Trond Myklebust <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/nfs/dir.c

···
 	struct dentry *parent = dget_parent(dentry);
 	struct inode *dir = d_inode(parent);
 	struct inode *inode;
+	struct dentry *d;
 	int error = -EACCES;
 
 	d_drop(dentry);
···
 			goto out_error;
 	}
 	inode = nfs_fhget(dentry->d_sb, fhandle, fattr, label);
-	error = PTR_ERR(inode);
-	if (IS_ERR(inode))
+	d = d_splice_alias(inode, dentry);
+	if (IS_ERR(d)) {
+		error = PTR_ERR(d);
 		goto out_error;
-	d_add(dentry, inode);
+	}
+	dput(d);
 out:
 	dput(parent);
 	return 0;