ib_core: More fixes to ib_sa_add_one error flow

ib_core: More fixes to ib_sa_add_one error flow

commit 0e7377eed fixed a resource leak of mad agents in
the ib_sa_add_one error flow.  However, the fix allowed
ib_mad_unregister_agent to be called in a case where the
ib_mad_register_agent request failed (resulting in an
illegal pointer in the agent field).  This caused a kernel
Oops in the error flow.

Fix this by calling ib_unregister_mad_agent only for cases where
ib_register_mad_agent succeeded.

In addition, separate the ib_register_event_handler() call error
flow from the loop error flow. If the call to
ib_register_event_handler fails, the client data must be reset
to NULL, (in case at some point ib_register_event_handler() is
modified so that it may return a non-zero (error) value).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
(Ported from Mellanox OFED 2.4)

Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>