RDMA/rxe: Fix "Replace mr by rkey in responder resources"

RDMA/rxe: Fix "Replace mr by rkey in responder resources"

The referenced commit generates a reference counting error if the rkey has
the same index but the wrong key. In this case the reference taken by
rxe_pool_get_index() is not dropped.

Drop the reference if the keys don't match in rxe_recheck_mr().  Check
that the mw and mr are still valid.

Fixes: 8a1a0be894da ("RDMA/rxe: Replace mr by rkey in responder resources")
Link: https://lore.kernel.org/r/20220411030647.20011-1-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>