xsk: Fix generic transmit when completion queue reservation fails

Two points of potential failure in the generic transmit function are:

  1. completion queue (cq) reservation failure.
  2. skb allocation failure

Originally the cq reservation was performed first, followed by the skb
allocation. Commit 67571640 ("xdp: fix possible cq entry leak")
reversed the order because at the time there was no mechanism available
to undo the cq reservation which could have led to possible cq entry leaks
in the event of skb allocation failure. However if the skb allocation is
performed first and the cq reservation then fails, the xsk skb destructor
is called which blindly adds the skb address to the already full cq leading
to undefined behavior.

This commit restores the original order (cq reservation followed by skb
allocation) and uses the xskq_prod_cancel helper to undo the cq reserve
in event of skb allocation failure.

Fixes: 67571640 ("xdp: fix possible cq entry leak")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220614070746.8871-1-ciara.loftus@intel.com