tcp: do not set a zero size receive buffer

The nipa CI is reporting frequent failures in the mptcp_connect
self-tests.

In the failing scenarios (TCP -> MPTCP) the involved sockets are
actually plain TCP ones, as fallback for passive socket at 2whs
time cause the MPTCP listener to actually create a TCP socket.

The transfer is stuck due to the receiver buffer being zero.
With the stronger check in place, tcp_clamp_window() can be invoked
while the TCP socket has sk_rmem_alloc == 0, and the receive buffer
will be zeroed, too.

Check for the critical condition in tcp_prune_queue() and just
drop the packet without shrinking the receiver buffer.

Fixes: 1d2fbaad ("tcp: stronger sk_rcvbuf checks")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20c18165d3f848e1c5c1b782d88c1a5ab38b3f70.1753118029.git.pabeni@redhat.com


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