IB/core: Add gid attributes to sysfs

This patch set adds attributes of net device and gid type to each GID
in the GID table. Users that use verbs directly need to specify
the GID index. Since the same GID could have different types or
associated net devices, users should have the ability to query the
associated GID attributes. Adding these attributes to sysfs.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>