NFSv4: Clean up nfs4_atomic_open

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>