IB/ipath: sysfs and ipathfs support for core driver

The ipathfs filesystem contains files that are not appropriate for
sysfs, because they contain binary data.  The hierarchy is simple; the
top-level directory contains driver-wide attribute files, while numbered
subdirectories contain per-device attribute files.

Our userspace code currently expects this filesystem to be mounted on
/ipathfs, but a final location has not yet been chosen.
Signed-off-by: Bryan O'Sullivan <bos@pathscale.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>