nfsd: add defines for NFSv4.2 extended attribute support

Add defines for server-side extended attribute support. Most have
already been added as part of client support, but these are
the network order error codes for the noxattr and xattr2big errors,
and the addition of the xattr support to the supported file
attributes (if configured).

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>