gNFS: allow truncate() from SETATTR over NFS for owner

NFSv3 does not have a TRUNCATE procedure, instead it is part of the
SETATTR (change the 'size' attribute). SETATTR with a new 'size'
succeeds on other NFS-servers, even when the owner of the file does not
have write permissions. Make Gluster/NFS behave the same way, by
checking if the RPC/pid comes from the NFS-server, and allow truncate()
when the file is owned by the user calling SETATTR.

BUG: 955753
Change-Id: I4b7cb8efe5a2032c6cd2eef6af610032f76d8b39
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/8889
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: soumya k <skoduri@redhat.com>