- fix writing to coda files. this is probably not the right way to do
this, but it satisfies the locking protocol:
1. Sometimes coda_open() is called with an unlocked vnode which
does not satisfy the locking protocol. Lock it for now. We
need to find out why this happens
2. VFS_VGET sometimes returns the container vnode unlocked. What
is the locking protocol for VFS_VGET? We also lock it here.