xprtrdma: Clear receive-side ownership pointers on release

Three small ownership-state cleanups land the transport in a
state that lets future reviewers reason about each pointer
locally rather than tracing the whole reply path:

rpcrdma_rep_put() clears rep->rr_rqst before the rep enters
rb_free_reps so that no rep on the free list still carries a
stale rqst pointer.  rpcrdma_reply_handler() and
rpcrdma_unpin_rqst() are the only sites that set rr_rqst;
rpcrdma_reply_handler() hands the rep through
rpcrdma_rep_put(), and rpcrdma_unpin_rqst() NULLs rr_rqst
directly because its error path abandons the rep for
teardown cleanup rather than returning it to rb_free_reps.

rpcrdma_reply_put() NULLs req->rl_reply before calling
rpcrdma_rep_put().  The previous order placed the rep on
rb_free_reps while req->rl_reply still pointed at it; the
window was harmless because xprt_rdma_free_slot() holds the
req exclusively across the pair, but closing it makes the
invariant 'rep on rb_free_reps implies no req references it'
strictly checkable.

rpcrdma_sendctx_unmap() and rpcrdma_sendctx_cancel() clear
req->rl_sendctx after dropping the sendctx pointer in the
sendctx ring.  Without this, req->rl_sendctx survives across
Send completion and points at a sendctx that may already have
been reassigned by rpcrdma_sendctx_get_locked() to a different
req.  No caller dereferences the stale pointer today --
rpcrdma_prepare_send_sges() overwrites it before the next
Send -- but a NULL is a more honest representation of 'the
Send is no longer outstanding' and lets the assertion patch
that follows trip on any future regression.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

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