rpc_pipe: rpc_dir_inode_operations can be static

Hi Jeff,

FYI, there are new sparse warnings show up in

tree:   git://git.linux-nfs.org/projects/trondmy/linux-nfs.git nfs-for-next
head:   296afe1f58d55fd56ed85daaafafcfee39f59ece
commit: 76fa6665

 [2/5] rpc_pipe: set dentry operations at d_alloc time

>> net/sunrpc/rpc_pipe.c:496:31: sparse: symbol 'rpc_dir_inode_operations' was not declared. Should it be static?

Please consider folding the attached diff :-)

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>