RDMA: Change MAD processing function to remove extra casting and parameter

All users of process_mad() converts input pointers from ib_mad_hdr to be
ib_mad, update the function declaration to use ib_mad directly.

Also remove not used input MAD size parameter.

Link: https://lore.kernel.org/r/20191029062745.7932-17-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Tested-By: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>