NFS: handle inode==NULL in __put_nfs_open_context

inode may be NULL when put_nfs_open_context is called from nfs_atomic_lookup
before d_add_unique(dentry, inode)

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>