iwlwifi: mvm: fix a use-after-free bug in iwl_mvm_tx_tso_segment

Accessing the hdr of an skb that was consumed already isn't
a good idea.
First ask if the skb is a QoS packet, then keep that data
on stack, and then consume the skb.
This was spotted by KASAN.

Cc: stable@vger.kernel.org
Fixes: 08f7d8b6 ("iwlwifi: mvm: bring back mvm GSO code")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>