rxrpc: Fix oops due to non-existence of prealloc backlog struct

If an AF_RXRPC service socket is opened and bound, but calls are
preallocated, then rxrpc_alloc_incoming_call() will oops because the
rxrpc_backlog struct doesn't get allocated until the first preallocation is
made.

Fix this by returning NULL from rxrpc_alloc_incoming_call() if there is no
backlog struct.  This will cause the incoming call to be aborted.

Reported-by: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
Suggested-by: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: LePremierHomme <kwqcheii@proton.me>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Willy Tarreau <w@1wt.eu>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250708211506.2699012-3-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>