nfscommon: Prepare the NFS common code for named attributes

Commit 2ec2ba7e232d added support for Solaris style extended
attributes (called named attributes, which is the NFSv4
terminology) to the VFS.

This patch updates the NFS code common to the NFS client and
server, to prepare it for the addition of named attribute
support.

The actual support for named attributes in the NFSv4 client
and server will be done in future commits.