NFS: Introduce iovec I/O helpers to fs/nfs/direct.c

Add helpers that iterate over multi-segment iovecs.  These will
be used to support multi-segment scatter/gather direct I/O in a
later patch.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>