NFS: Fix the ftruncate() credential problem

ftruncate() access checking is supposed to be performed at open() time,
just like reads and writes.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

diff --cc fs/nfs/inode.c
Simple merge

diff --cc fs/nfs/nfs3proc.c
Simple merge

diff --cc fs/nfs/nfs4proc.c
Simple merge

diff --cc fs/nfs/proc.c
Simple merge