[ofa-general] removing PR caching from IPoIB

[Roland Dreier]

 > IPoIB permanently caches PRs with no mechanism in place to keep them
 > in sync with the SA.  The goal is to limit the PR lifetime to prevent
 > the use of stale paths.
 > 
 > A side benefit of limiting the PR scope to a neighbour is that
 > different neighbours could use different paths, versus using a single
 > path per DGID.

OK, that makes sense.  However, I don't think we want to do a path
record lookup for every unicast ARP send, right?  So I think we have
to have some table of DGID -> AH mappings independent of the
ipoib_neigh stuff.

Adding a reference count to the path structure as you mentioned before
makes sense to me.  The only slightly ugly thing is handling dropping
the reference when an ARP send completes, but I guess that should be
possible to do somehow.  Or were you suggesting using the existing AH
reference counting and letting that handle things?  That makes sense
to me and actually seems pretty elegant.

 - R.