nfsv4: Add support to NFSv4 for named attributes

NFSv4 supports a feature called named attributes, that are
essentially Solaris style extended attributes.
Commits starting with 2ec2ba7e232d added Solaris style extended
attribute support.

This patch uses the Solaris style extended attribute
support to provide support for NFSv4.
Since nfsv4_loadattr() needed an additional argument,
many file are affected, although many in a trivial way.

For the NFSv4 server to support named attributes, ZFS must
be patched and only ZFS file systems support these Solaris
style extended attributes.