Add a check for MNTK_UNMOUNTF at the beginning of nfs_sync()
in the old NFS client so that a forced dismount doesn't
get stuck in the VFS_SYNC() call that happens before
VFS_UNMOUNT() in dounmount(). Analagous to r222329 for the new NFS client.
An additional change is needed before forced dismounts will work.

PR: kern/157365
MFC after: 2 weeks