RDMA/rxe: Stop lookup of partially built objects

RDMA/rxe: Stop lookup of partially built objects

Currently the rdma_rxe driver has a security weakness due to giving
objects which are partially initialized indices allowing external actors
to gain access to them by sending packets which refer to their
index (e.g. qpn, rkey, etc) causing unpredictable results.

This patch adds a new API rxe_finalize(obj) which enables looking up pool
objects from indices using rxe_pool_get_index() for AH, QP, MR, and
MW. They are added in create verbs only after the objects are fully
initialized.

It also adds wait for completion to destroy/dealloc verbs to assure that
all references have been dropped before returning to rdma_core by
implementing a new rxe_pool API rxe_cleanup() which drops a reference to
the object and then waits for all other references to be dropped.  When
the last reference is dropped the object is completed by kref.  After that
it cleans up the object and if locally allocated frees the memory. In the
special case of address handle objects the delay is implemented separately
if the destroy_ah call is not sleepable.

Combined with deferring cleanup code to type specific cleanup routines
this allows all pending activity referring to objects to complete before
returning to rdma_core.

Link: https://lore.kernel.org/r/20220612223434.31462-2-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>