new primitive: discard_new_inode()

We don't want open-by-handle picking half-set-up in-core
struct inode from e.g. mkdir() having failed halfway through.
In other words, we don't want such inodes returned by iget_locked()
on their way to extinction.  However, we can't just have them
unhashed - otherwise open-by-handle immediately *after* that would've
ended up creating a new in-core inode over the on-disk one that
is in process of being freed right under us.

	Solution: new flag (I_CREATING) set by insert_inode_locked() and
removed by unlock_new_inode() and a new primitive (discard_new_inode())
to be used by such halfway-through-setup failure exits instead of
unlock_new_inode() / iput() combinations.  That primitive unlocks new
inode, but leaves I_CREATING in place.

	iget_locked() treats finding an I_CREATING inode as failure
(-ESTALE, once we sort out the error propagation).
	insert_inode_locked() treats the same as instant -EBUSY.
	ilookup() treats those as icache miss.

[Fix by Dan Carpenter <dan.carpenter@oracle.com> folded in]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -1892,7 +1892,7 @@ void d_instantiate_new(struct dentry *entry, struct inode *inode)
 	spin_lock(&inode->i_lock);
 	__d_instantiate(entry, inode);
 	WARN_ON(!(inode->i_state & I_NEW));
-	inode->i_state &= ~I_NEW;
+	inode->i_state &= ~I_NEW & ~I_CREATING;
 	smp_mb();
 	wake_up_bit(&inode->i_state, __I_NEW);
 	spin_unlock(&inode->i_lock);