chelsio/chtls: fix a double free in chtls_setkey()

The "skb" is freed by the transmit code in cxgb4_ofld_send() and we
shouldn't use it again.  But in the current code, if we hit an error
later on in the function then the clean up code will call kfree_skb(skb)
and so it causes a double free.

Set the "skb" to NULL and that makes the kfree_skb() a no-op.

Fixes: d25f2f71 ("crypto: chtls - Program the TLS session Key")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X8ilb6PtBRLWiSHp@mwanda


Signed-off-by: Jakub Kicinski <kuba@kernel.org>