IB/core: Reduce the places that use zgid

Instead of open coding memcmp() to check whether a given GID is zero or
not, use a helper function to do so, and replace instances of
memcpy(z,&zgid) with memset.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/rdma/ib_cache.h

@@ -149,4 +149,5 @@ int ib_get_cached_port_state(struct ib_device *device,
 			      u8                port_num,
 			      enum ib_port_state *port_active);
 
+bool rdma_is_zero_gid(const union ib_gid *gid);
 #endif /* _IB_CACHE_H */