rxrpc: Replace fake flex-array with flexible-array member

Zero-length arrays as fake flexible arrays are deprecated and we are
moving towards adopting C99 flexible-array members instead.

Transform zero-length array into flexible-array member in struct
rxrpc_ackpacket.

Address the following warnings found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
net/rxrpc/call_event.c:149:38: warning: array subscript i is outside array bounds of ‘uint8_t[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/263
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html

 [1]
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>