NFS: Don't abuse NFS_INO_REVAL_FORCED in nfs_post_op_update_inode_locked()

The NFS_INO_REVAL_FORCED flag now really only has meaning for the
case when we've just been handed a delegation for a file that was already
cached, and we're unsure about that cache.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>