The old utrace-uprobes operated on a per-process model.  We would use
task_finder to locate processes with the interesting module, then after
verifying the buildid, register a uprobe in that process.

The new inode-uprobes operates on an inode model, systemwide.  I argued that it
doesn't make sense to force the per-process model on this, because you can't
really register a uprobe for just one process.  So instead we're looking up and
registering on inodes when stap.ko starts.  But the downside is that the
buildid check doesn't have any chance to validate this, since the inode may not
even be in memory yet.

So here's a proposed compromise -- delay registration, but still treat it as
systemwide.  This may look roughly like:
(1) On stap.ko start, just register task_finder for each stapiu_target.
(2) The first time a target is seen, check its buildid.  If it matches, then
igrab that inode and register every stapiu_consumer for that target.
(3) Every time the target is seen, still need to manipulate any semaphores.
(4) On stap.ko unload, unregister whatever consumers came alive, and iput all
the targets we did igrab.

Note we don't need to unregister+iput targets when individual processes go
away, because the registration is systemwide, and we know that we have an
inode* with a good buildid.