NFS: Fix a busy inodes issue...

The nfs_open_context may live longer than the file descriptor that spawned
it, so it needs to carry a reference to the vfsmount. If not, then
generic_shutdown_super() may end up being called before reads and writes
have been flushed out.

Make a couple of functions static while we're at it...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>