SUNRPC: pipefs dentry lookup helper introduced

In all places, where pipefs dentries are created, only directory inode is
actually required to create new dentry. And all this directories has root
pipefs dentry as their parent. So we actually don't need this pipefs mount
point at all if some pipefs lookup method will be provided.
IOW, all we really need is just superblock and simple lookup method to find
root's child dentry with appropriate name. And this patch introduces this
method.
Note, that no locking implemented in rpc_d_lookup_sb(). So it can be used only
in case of assurance, that pipefs superblock still exist. IOW, we can use this
method only in pipefs mount-umount notification subscribers callbacks.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>