Retrying revoke()

I'm curious about revoke. For forced drm module unloading of drivers (nouveau/ati) I've done something similar, but the real problem is racing with mmaps, during mmap and munmap there is a point where the mapping is removed off (or not yet added) to the list, but is still valid, just not yet tracked.

I haven't figured out how to close the mmap race there, but for the revoke case it might be important.