rxrpc: Fix key quota calculation for multitoken keys

rxrpc: Fix key quota calculation for multitoken keys

In the rxrpc key preparsing, every token extracted sets the proposed quota
value, but for multitoken keys, this will overwrite the previous proposed
quota, losing it.

Fix this by adding to the proposed quota instead.

Fixes: 8a7a3eb4ddbe ("KEYS: RxRPC: Use key preparsing")
Closes: https://sashiko.dev/#/patchset/20260319150150.4189381-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260408121252.2249051-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>