NFS: Remove redundant open context from nfs_page

The lock context already references and tracks the open context, so
take the opportunity to save some space in struct nfs_page.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>