From: NeilBrown <neilb@cse.unsw.edu.au>

The credentials (uid/gid) of a process are set when a filehandle is
verified.  Nfsv4 allows requests without an explicit filehandle (instead,
an implicit 'root' filehandle) so we much make sure the credentials are set
for these requests too.

From: "J. Bruce Fields" <bfields@fieldses.org>

From: Andros: added a call to nfsd_setuser in nfsd4_putrootfh so that nfsd
runs as the rpc->cred user.