We shouldn't set real_dev to NULL because packets can be in transit and
xfrm might call xdo_dev_offload_ok() in parallel. All callbacks assume
real_dev is set.

 Example trace:
 kernel: BUG: unable to handle page fault for address: 0000000000001030
 kernel: bond0: (slave eni0np1): making interface the new active one
 kernel: #PF: supervisor write access in kernel mode
 kernel: #PF: error_code(0x0002) - not-present page
 kernel: PGD 0 P4D 0
 kernel: Oops: 0002 [#1] PREEMPT SMP
 kernel: CPU: 4 PID: 2237 Comm: ping Not tainted 6.7.7+ #12
 kernel: Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-2.fc40 04/01/2014
 kernel: RIP: 0010:nsim_ipsec_offload_ok+0xc/0x20 [netdevsim]
 kernel: bond0: (slave eni0np1): bond_ipsec_add_sa_all: failed to add SA
 kernel: Code: e0 0f 0b 48 83 7f 38 00 74 de 0f 0b 48 8b 47 08 48 8b 37 48 8b 78 40 e9 b2 e5 9a d7 66 90 0f 1f 44 00 00 48 8b 86 80 02 00 00 <83> 80 30 10 00 00 01 b8 01 00 00 00 c3 0f 1f 80 00 00 00 00 0f 1f
 kernel: bond0: (slave eni0np1): making interface the new active one
 kernel: RSP: 0018:ffffabde81553b98 EFLAGS: 00010246
 kernel: bond0: (slave eni0np1): bond_ipsec_add_sa_all: failed to add SA
 kernel:
 kernel: RAX: 0000000000000000 RBX: ffff9eb404e74900 RCX: ffff9eb403d97c60
 kernel: RDX: ffffffffc090de10 RSI: ffff9eb404e74900 RDI: ffff9eb3c5de9e00
 kernel: RBP: ffff9eb3c0a42000 R08: 0000000000000010 R09: 0000000000000014
 kernel: R10: 7974203030303030 R11: 3030303030303030 R12: 0000000000000000
 kernel: R13: ffff9eb3c5de9e00 R14: ffffabde81553cc8 R15: ffff9eb404c53000
 kernel: FS:  00007f2a77a3ad00(0000) GS:ffff9eb43bd00000(0000) knlGS:0000000000000000
 kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 kernel: CR2: 0000000000001030 CR3: 00000001122ab000 CR4: 0000000000350ef0
 kernel: bond0: (slave eni0np1): making interface the new active one
 kernel: Call Trace:
 kernel:  <TASK>
 kernel:  ? __die+0x1f/0x60
 kernel: bond0: (slave eni0np1): bond_ipsec_add_sa_all: failed to add SA
 kernel:  ? page_fault_oops+0x142/0x4c0
 kernel:  ? do_user_addr_fault+0x65/0x670
 kernel:  ? kvm_read_and_reset_apf_flags+0x3b/0x50
 kernel: bond0: (slave eni0np1): making interface the new active one
 kernel:  ? exc_page_fault+0x7b/0x180
 kernel:  ? asm_exc_page_fault+0x22/0x30
 kernel:  ? nsim_bpf_uninit+0x50/0x50 [netdevsim]
 kernel: bond0: (slave eni0np1): bond_ipsec_add_sa_all: failed to add SA
 kernel:  ? nsim_ipsec_offload_ok+0xc/0x20 [netdevsim]
 kernel: bond0: (slave eni0np1): making interface the new active one
 kernel:  bond_ipsec_offload_ok+0x7b/0x90 [bonding]
 kernel:  xfrm_output+0x61/0x3b0
 kernel: bond0: (slave eni0np1): bond_ipsec_add_sa_all: failed to add SA
 kernel:  ip_push_pending_frames+0x56/0x80

Fixes: 18cb261a ("bonding: support hardware encryption offload to slaves")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>