IB/core: Add helpers for uncached GID and P_Key searches

Add ib_find_gid() and ib_find_pkey() functions that use uncached device
queries.  The calls might block but the returns are always up-to-date.
Cache P_Key and GID table lengths in core to avoid extra port info queries.

Signed-off-by: Yosef Etigin <yosefe@voltaire.com>
Acked-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>