RDMA/core: Share driver structure size with core

Add new macros to be used in drivers while registering ops structure and
IB/core while calling allocation routines, so drivers won't need to
perform kzalloc/kfree in their paths.

The change in allocation stage allows us to initialize common fields prior
to calling to drivers (e.g. restrack).

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