NFS: Fix a race in sillyrename

lookup() and sillyrename() can race one another because the sillyrename()
completion cannot take the parent directory's inode->i_mutex since the
latter may be held by whoever is calling dput().

We therefore have little option but to add extra locking to ensure that
nfs_lookup() and nfs_atomic_open() do not race with the sillyrename
completion.
If somebody has looked up the sillyrenamed file in the meantime, we just
transfer the sillydelete information to the new dentry.

Please refer to the bug-report at
	http://bugzilla.linux-nfs.org/show_bug.cgi?id=150

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>