s390/ism: Fix UAF of sba and ieq during ism_dev_exit()

s390/ism: Fix UAF of sba and ieq during ism_dev_exit()

A ism interrupt handler can be active in parallel with ism_dev_exit(),
accessing freed data structures.

No new interrupts will be generated after unregister_ieq(). Drain ongoing
interrupt handlers by free_irq(), before freeing ism data structures.

Fixes: 684b89bc39ce ("s390/ism: add device driver for internal shared memory")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20260805131043.954639-1-wintera@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>