NFSv4.2: hook in the user extended attribute handlers

Now that all the lower level code is there to make the RPC calls, hook
it in to the xattr handlers and the listxattr entry point, to make them
available.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>