net/af_iucv: fix NULL deref in afiucv_hs_callback_syn()

net/af_iucv: fix NULL deref in afiucv_hs_callback_syn()

afiucv_hs_callback_syn() allocates the child socket with GFP_ATOMIC.
If the allocation fails, nsk is NULL.

The connection-refused path is entered when the listen state check
fails, the accept backlog is full, or nsk is NULL. The code
unconditionally calls iucv_sock_kill(nsk) in that path.

iucv_sock_kill() does not accept a NULL socket pointer and immediately
dereferences sk via sock_flag(sk, SOCK_ZAPPED). When nsk is NULL,
calling iucv_sock_kill(nsk) results in a NULL pointer dereference.

Only call iucv_sock_kill() when a child socket was successfully
allocated.

Fixes: 3881ac441f64 ("af_iucv: add HiperSockets transport")
Cc: stable@vger.kernel.org
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Hidayath Khan <hidayath@linux.ibm.com>
Link: https://patch.msgid.link/20260709191732.124092-1-hidayath@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>