nfs: per-name sillyunlink exclusion

use d_alloc_parallel() for sillyunlink/lookup exclusion and
explicit rwsem (nfs_rmdir() being a writer and nfs_call_unlink() -
a reader) for rmdir/sillyunlink one.

That ought to make lookup/readdir/!O_CREAT atomic_open really
parallel on NFS.

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

fs/nfs/nfstrace.h

@@ -702,7 +702,7 @@ TRACE_EVENT(nfs_sillyrename_unlink,
 		),
 
 		TP_fast_assign(
-			struct inode *dir = data->dir;
+			struct inode *dir = d_inode(data->dentry->d_parent);
 			size_t len = data->args.name.len;
 			__entry->dev = dir->i_sb->s_dev;
 			__entry->dir = NFS_FILEID(dir);