ath11k: Reset ath11k_skb_cb before setting new flags

It was observed that the codepath for the ATH11K_SKB_HW_80211_ENCAP was
used even when the IEEE80211_TX_CTRL_HW_80211_ENCAP was not enabled for a
an skbuff. This became even more prominent when the QCAs wlan-open patchset
for ath11k [1] was applied and a sane looking fix just caused crashes when
injecting frames via a monitor interface (for example with ratechecker):

  [   86.963152] Unable to handle kernel NULL pointer dereference at virtual address 00000338
  [   86.963192] pgd = ffffffc0008f0000
  [   86.971034] [00000338] *pgd=0000000051706003, *pud=0000000051706003, *pmd=0000000051707003, *pte=00e800000b000707
  [   86.984292] Internal error: Oops: 96000006 [#1] PREEMPT SMP
  [...]
  [   87.713339] [<ffffffbffc802480>] ieee80211_tx_status_8023+0xf8/0x220 [mac80211]
  [   87.715654] [<ffffffbffc98bad4>] ath11k_dp_tx_completion_handler+0x42c/0xa10 [ath11k]
  [   87.722924] [<ffffffbffc989190>] ath11k_dp_service_srng+0x70/0x3c8 [ath11k]
  [   87.730831] [<ffffffbffca03460>] 0xffffffbffca03460
  [   87.737599] [<ffffffc00046ef58>] net_rx_action+0xf8/0x288
  [   87.742462] [<ffffffc000097554>] __do_softirq+0xfc/0x220
  [   87.748014] [<ffffffc000097900>] irq_exit+0x98/0xe8
  [   87.753396] [<ffffffc0000cf188>] __handle_domain_irq+0x90/0xb8
  [   87.757999] [<ffffffc000081ca4>] gic_handle_irq+0x6c/0xc8
  [   87.763899] Exception stack(0xffffffc00081bdc0 to 0xffffffc00081bef0)

Problem is that the state of ath11k_skb_cb->flags must be considered
unknown and could contain anything when it is not manually initialized. So
it could also contain ATH11K_SKB_HW_80211_ENCAP. And this can result in the
code to assume that the ath11k_skb_cb->vif is set - even when this is not
always the case for non ATH11K_SKB_HW_80211_ENCAP transmissions.

Tested-on: IPQ8074 hw2.0 WLAN.HK.2.4.0.1.r1-00026-QCAHKSWPL_SILICONZ-2

[1] https://source.codeaurora.org/quic/qsdk/oss/system/feeds/wlan-open/tree/mac80211/patches?h=NHSS.QSDK.11.4.r3
    (162 patches at the moment which are often not upstreamed but essential
     to get ath11k working)

Fixes: e7f33e0c ("ath11k: add tx hw 802.11 encapsulation offloading support")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201119154235.263250-2-sven@narfation.org

drivers/net/wireless/ath/ath11k/mac.c

@@ -4077,6 +4077,7 @@ static void ath11k_mac_op_tx(struct ieee80211_hw *hw,
 	bool is_prb_rsp;
 	int ret;
 
+	memset(skb_cb, 0, sizeof(*skb_cb));
 	skb_cb->vif = vif;
 
 	if (key) {