bonding: fix xfrm real_dev null pointer dereference

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

bonding: fix xfrm real_dev null pointer dereference

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