NFSD: Change name of extended attribute containing junction

As of fedfs-utils-0.8.0, user space stores all NFS junction
information in a single extended attribute: "trusted.junction.nfs".

Both FedFS and NFS basic junctions are stored in this one attribute,
and the intention is that all future forms of NFS junction metadata
will be stored in this attribute.  Other protocols may use a different
extended attribute.

Thus NFSD needs to look only for that one extended attribute.  The
"trusted.junction.type" xattr is deprecated.  fedfs-utils-0.8.0 will
continue to attach a "trusted.junction.type" xattr to junctions, but
future fedfs-utils releases may no longer do that.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>