Btrfs: only clear the need lookup flag after the dentry is setup

We can race with readdir and the RCU path walking stuff.  This is because we
clear the need lookup flag before actually instantiating the inode.  This will
lead the RCU path walk stuff to find a dentry it thinks is valid without a
d_inode attached.  So instead unhash the dentry when we first start the lookup,
and then clear the flag after we've instantiated the dentry so we're garunteed
to either try the slow lookup, or have the d_inode set properly.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>