NFS: remove BUG possibility in nfs4_open_and_get_state

commit 4fa2c54b5198d09607a534e2fd436581064587ed
    NFS: nfs4_do_open should add negative results to the dcache.

used "d_drop(); d_add();" to ensure that a dentry was hashed
as a negative cached entry.
This is not safe if the dentry has an non-NULL ->d_inode.
It will trigger a BUG_ON in d_instantiate().
In that case, d_delete() is needed.

Also, only d_add if the dentry is currently unhashed, it seems
pointless removed and re-adding it unchanged.

Reported-by: Christoph Hellwig <hch@infradead.org>
Fixes: 4fa2c54b5198d09607a534e2fd436581064587ed
Cc: Jeff Layton <jeff.layton@primarydata.com>
Link: http://lkml.kernel.org/r/20140908144525.GB19811@infradead.org
Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4proc.c

@@ -2226,9 +2226,13 @@ static int _nfs4_open_and_get_state(struct nfs4_opendata *opendata,
 	ret = _nfs4_proc_open(opendata);
 	if (ret != 0) {
 		if (ret == -ENOENT) {
-			d_drop(opendata->dentry);
-			d_add(opendata->dentry, NULL);
-			nfs_set_verifier(opendata->dentry,
+			dentry = opendata->dentry;
+			if (dentry->d_inode)
+				d_delete(dentry);
+			else if (d_unhashed(dentry))
+				d_add(dentry, NULL);
+
+			nfs_set_verifier(dentry,
 					 nfs_save_change_attribute(opendata->dir->d_inode));
 		}
 		goto out;