Implement the main entry points for the *XATTR operations.

Add functions to calculate the reply size for the user extended attribute
operations, and implement the XDR encode / decode logic for these
operations.

Add the user extended attributes operations to nfsd4_ops.
Signed-off-by: NFrank van der Linden <fllinden@amazon.com>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>