rxrpc: Fix over large frame size warning

rxrpc: Fix over large frame size warning

Under some circumstances, the compiler will emit the following warning for
rxrpc_send_response():

   net/rxrpc/output.c: In function 'rxrpc_send_response':
   net/rxrpc/output.c:974:1: warning: the frame size of 1160 bytes is larger than 1024 bytes

This occurs because the local variables include a 16-element scatterlist
array and a 16-element bio_vec array.  It's probably not actually a problem
as this function is only called by the rxrpc I/O thread function in a
kernel thread and there won't be much on the stack before it.

Fix this by overlaying the bio_vec array over the kvec array in the
rxrpc_local struct.  There is one of these per I/O thread and the kvec
array is intended for pointing at bits of a packet to be transmitted,
typically a DATA or an ACK packet.  As packets for a local endpoint are
only transmitted by its specific I/O thread, there can be no race, and so
overlaying this bit of memory should be no problem.

Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506240423.E942yKJP-lkp@intel.com/
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/20250707102435.2381045-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>