rxrpc: Fix call state set to not include the SERVER_SECURING state

rxrpc: Fix call state set to not include the SERVER_SECURING state

The RXRPC_CALL_SERVER_SECURING state doesn't really belong with the other
states in the call's state set as the other states govern the call's Rx/Tx
phase transition and govern when packets can and can't be received or
transmitted.  The "Securing" state doesn't actually govern the reception of
packets and would need to be split depending on whether or not we've
received the last packet yet (to mirror RECV_REQUEST/ACK_REQUEST).

The "Securing" state is more about whether or not we can start forwarding
packets to the application as recvmsg will need to decode them and the
decoding can't take place until the challenge/response exchange has
completed.

Fix this by removing the RXRPC_CALL_SERVER_SECURING state from the state
set and, instead, using a flag, RXRPC_CALL_CONN_CHALLENGING, to track
whether or not we can queue the call for reception by recvmsg() or notify
the kernel app that data is ready.  In the event that we've already
received all the packets, the connection event handler will poke the app
layer in the appropriate manner.

Also there's a race whereby the app layer sees the last packet before rxrpc
has managed to end the rx phase and change the state to one amenable to
allowing a reply.  Fix this by queuing the packet after calling
rxrpc_end_rx_phase().

Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250204230558.712536-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>