driver/linux_onload: fix RCU usage

[ol-alexandra]

Call rcu_read_lock() becore rcu_assign_pointer() in oo_fd_replace_file().

With this patch debug kernel stops from complaining on

[ 2863.017775] oo:ta_rpcs[34810]: Sharing Onload 4b6ff62  [0,sapi-ts]
[ 2863.030226] 
[ 2863.032853] =============================
[ 2863.037467] WARNING: suspicious RCU usage
[ 2863.042065] 6.8.12-1-debug-generic #2 Tainted: G           OE     
[ 2863.049105] -----------------------------
[ 2863.053686] /srv/testers/tmp/session_2024.11.21/balin_oo_i40e_6.8.12-1-debug-generic_x86_64_ndebug/src/driver/linux_onload/ossock_calls.c:126 suspicious rcu_dereference_check() usage!
[ 2863.072325] 
[ 2863.072325] other info that might help us debug this:
[ 2863.072325] 
[ 2863.086305] 
[ 2863.086305] rcu_scheduler_active = 2, debug_locks = 1
[ 2863.096900] 1 lock held by ta_rpcs/34810:
[ 2863.103073]  #0: ffff888146640e18 (&p->alloc_lock){+.+.}-{3:3}, at: oo_fd_replace_file+0x36/0x5f0 [onload]
[ 2863.115748] 
[ 2863.115748] stack backtrace:
[ 2863.123921] CPU: 0 PID: 34810 Comm: ta_rpcs Tainted: G           OE      6.8.12-1-debug-generic #2
[ 2863.135666] Hardware name: Dell Inc. PowerEdge R230/0FRVY0, BIOS 2.3.2 11/16/2017
[ 2863.145745] Call Trace:
[ 2863.150112]  <TASK>
[ 2863.154068]  dump_stack_lvl+0xeb/0x100
[ 2863.159889]  dump_stack+0x10/0x20
[ 2863.165180]  lockdep_rcu_suspicious+0x21e/0x410
[ 2863.171835]  oo_fd_replace_file+0x52f/0x5f0 [onload]
[ 2863.178982]  efab_tcp_helper_handover+0x5ec/0xb00 [onload]
[ 2863.186696]  ? __pfx_lock_release+0x10/0x10
[ 2863.193336]  ? __pfx_efab_tcp_helper_handover+0x10/0x10 [onload]
[ 2863.201639]  ? __might_fault+0xd9/0x1a0
[ 2863.207459]  oo_fop_unlocked_ioctl+0x1bf/0x470 [onload]
[ 2863.214861]  ? __pfx_oo_fop_unlocked_ioctl+0x10/0x10 [onload]
[ 2863.222844]  ? debug_smp_processor_id+0x17/0x30
[ 2863.229423]  ? do_syscall_64+0xa1/0x190
[ 2863.235212]  ? __kasan_check_read+0x11/0x20
[ 2863.241356]  ? __fget_light+0x59/0x200
[ 2863.247003]  __x64_sys_ioctl+0x140/0x1c0
[ 2863.252848]  x64_sys_call+0x1395/0x2660
[ 2863.258573]  do_syscall_64+0x95/0x190
[ 2863.264094]  ? irqentry_exit+0x77/0xb0
[ 2863.269697]  ? exc_page_fault+0x92/0x140
[ 2863.275464]  entry_SYSCALL_64_after_hwframe+0x78/0x80
[ 2863.282468] RIP: 0033:0x77b715f1a94f
[ 2863.287790] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
[ 2863.311685] RSP: 002b:00007ffd1001abf0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 2863.321608] RAX: ffffffffffffffda RBX: 000077b71602d0c8 RCX: 000077b715f1a94f
[ 2863.331034] RDX: 00007ffd1001ac54 RSI: 00000000c0045a39 RDI: 0000000000000006
[ 2863.340476] RBP: 0000000000000006 R08: 000056521de76320 R09: 000077b7137ff000
[ 2863.349941] R10: 000000005101c0fe R11: 0000000000000246 R12: 00000000c0045a39
[ 2863.359402] R13: 000077b715dff4e0 R14: 0000000000000000 R15: 000056521de6a8b0