Fix the NFSv2 extended attribute support to handle 0 length attributes.

I did not realize that zero length attributes are allowed, but they are.
This patch fixes the NFSv4.2 client and server to handle zero length
extended attributes correctly.

Submitted by: Frank van der Linden <fllinden@amazon.com> (earlier version)
Reported by: Frank van der Linden <fllinder@amazon.com>