This tracepoint can crash when dereferencing snd_task because
when some transports connect, they put a cookie in that field
instead of a pointer to an rpc_task.

BUG: KASAN: use-after-free in trace_event_raw_event_xprt_writelock_event+0x141/0x18e [sunrpc]
Read of size 2 at addr ffff8881a83bd3a0 by task git/331872

CPU: 11 PID: 331872 Comm: git Tainted: G S                5.12.0-rc2-00007-g3ab6e585a7f9 #1453
Hardware name: Supermicro SYS-6028R-T/X10DRi, BIOS 1.1a 10/16/2015
Call Trace:
 dump_stack+0x9c/0xcf
 print_address_description.constprop.0+0x18/0x239
 kasan_report+0x174/0x1b0
 trace_event_raw_event_xprt_writelock_event+0x141/0x18e [sunrpc]
 xprt_prepare_transmit+0x8e/0xc1 [sunrpc]
 call_transmit+0x4d/0xc6 [sunrpc]

Fixes: 9ce07ae5 ("SUNRPC: Replace dprintk() call site in xprt_prepare_transmit")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>