rxrpc: fix RESPONSE authenticator parser OOB read

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: fix RESPONSE authenticator parser OOB read

rxgk_verify_authenticator() copies auth_len bytes into a temporary
buffer and then passes p + auth_len as the parser limit to
rxgk_do_verify_authenticator(). Since p is a __be32 *, that inflates the
parser end pointer by a factor of four and lets malformed RESPONSE
authenticators read past the kmalloc() buffer.

Decoded from the original latest-net reproduction logs with
scripts/decode_stacktrace.sh:

BUG: KASAN: slab-out-of-bounds in rxgk_verify_response()
Call Trace:
dump_stack_lvl() [lib/dump_stack.c:123]
print_report() [mm/kasan/report.c:379 mm/kasan/report.c:482]
kasan_report() [mm/kasan/report.c:597]
rxgk_verify_response()
[net/rxrpc/rxgk.c:1103 net/rxrpc/rxgk.c:1167
net/rxrpc/rxgk.c:1274]
rxrpc_process_connection()
[net/rxrpc/conn_event.c:266 net/rxrpc/conn_event.c:364
net/rxrpc/conn_event.c:386]
process_one_work() [kernel/workqueue.c:3281]
worker_thread()
[kernel/workqueue.c:3353 kernel/workqueue.c:3440]
kthread() [kernel/kthread.c:436]
ret_from_fork() [arch/x86/kernel/process.c:164]

Allocated by task 54:
rxgk_verify_response()
[include/linux/slab.h:954 net/rxrpc/rxgk.c:1155
net/rxrpc/rxgk.c:1274]
rxrpc_process_connection()
[net/rxrpc/conn_event.c:266 net/rxrpc/conn_event.c:364
net/rxrpc/conn_event.c:386]

Convert the byte count to __be32 units before constructing the parser
limit.