IB/srpt: Fix memory leak in srpt_add_one

Failure in srpt_refresh_port() for the second port will leave MAD
registered for the first one, however, the srpt_add_one() will be marked
as "failed" and SRPT will leak resources for that registered but not used
and released first port.

Unregister the MAD agent for all ports in case of failure.

Fixes: a42d985b

 ("ib_srpt: Initial SRP Target merge for v3.3-rc1")
Link: https://lore.kernel.org/r/20201028065051.112430-1-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>