ibmvnic: Don't reference skb after sending to VIOS

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

ibmvnic: Don't reference skb after sending to VIOS

Previously, after successfully flushing the xmit buffer to VIOS,

the tx_bytes stat was incremented by the length of the skb.

It is invalid to access the skb memory after sending the buffer to

the VIOS because, at any point after sending, the VIOS can trigger

an interrupt to free this memory. A race between reading skb->len

and freeing the skb is possible (especially during LPM) and will

result in use-after-free:

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BUG: KASAN: slab-use-after-free in ibmvnic_xmit+0x75c/0x1808 [ibmvnic]

Read of size 4 at addr c00000024eb48a70 by task hxecom/14495

<...>

Call Trace:

[c000000118f66cf0] [c0000000018cba6c] dump_stack_lvl+0x84/0xe8 (unreliable)

[c000000118f66d20] [c0000000006f0080] print_report+0x1a8/0x7f0

[c000000118f66df0] [c0000000006f08f0] kasan_report+0x128/0x1f8

[c000000118f66f00] [c0000000006f2868] __asan_load4+0xac/0xe0

[c000000118f66f20] [c0080000046eac84] ibmvnic_xmit+0x75c/0x1808 [ibmvnic]

[c000000118f67340] [c0000000014be168] dev_hard_start_xmit+0x150/0x358

<...>

Freed by task 0:

kasan_save_stack+0x34/0x68

kasan_save_track+0x2c/0x50

kasan_save_free_info+0x64/0x108

__kasan_mempool_poison_object+0x148/0x2d4

napi_skb_cache_put+0x5c/0x194

net_tx_action+0x154/0x5b8

handle_softirqs+0x20c/0x60c

do_softirq_own_stack+0x6c/0x88

<...>

The buggy address belongs to the object at c00000024eb48a00 which

belongs to the cache skbuff_head_cache of size 224

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