IB/cache: Use correct pointer to calculate size

When allocating gid_cache, use kmalloc(sizeof *gid_cache, ...) rather
than kmalloc(sizeof *pkey_cache, ...).  It doesn't really matter which
one is used, since the size ends up the same either way, but it's much
better to say what we mean.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>