Start moving the 'struct nameidata' dependent code out of the lower level
NFS code in preparation for the removal of open intents.

Instead of the struct nameidata, we pass down a partially initialised
struct nfs_open_context that will be fully initialised by the atomic open
upon success.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>