[IPoIB] add path record information in debugfs

Add ibX_path files to debugfs that contain information about the IPoIB
path cache.  IPoIB ARP only gives GIDs, which the IPoIB driver must
resolve to real IB paths through the ib_sa module.  For debugging,
when the ARP table looks OK but traffic isn't flowing, it's useful to
be able to see if the resolution from GID to path worked.

Also clean up the formatting of the existing _mcg debugfs files.

Signed-off-by: Roland Dreier <rolandd@cisco.com>