s390/kprobes: fix irq mask clobbering on kprobe reenter from post_handler

Recent test_kprobe_missed kprobes kunit test uncovers the following error
(reported when CONFIG_DEBUG_ATOMIC_SLEEP is enabled):

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:580
in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 662, name: kunit_try_catch
preempt_count: 0, expected: 0
RCU nest depth: 0, expected: 0
no locks held by kunit_try_catch/662.
irq event stamp: 280
hardirqs last  enabled at (279): [<00000003e60a3d42>] __do_pgm_check+0x17a/0x1c0
hardirqs last disabled at (280): [<00000003e3bd774a>] kprobe_exceptions_notify+0x27a/0x318
softirqs last  enabled at (0): [<00000003e3c5c890>] copy_process+0x14a8/0x4c80
softirqs last disabled at (0): [<0000000000000000>] 0x0
CPU: 46 PID: 662 Comm: kunit_try_catch Tainted: G                 N 6.2.0-173644-g44c18d77f0c0 #2
Hardware name: IBM 3931 A01 704 (LPAR)
Call Trace:
 [<00000003e60a3a00>] dump_stack_lvl+0x120/0x198
 [<00000003e3d02e82>] __might_resched+0x60a/0x668
 [<00000003e60b9908>] __mutex_lock+0xc0/0x14e0
 [<00000003e60bad5a>] mutex_lock_nested+0x32/0x40
 [<00000003e3f7b460>] unregister_kprobe+0x30/0xd8
 [<00000003e51b2602>] test_kprobe_missed+0xf2/0x268
 [<00000003e51b5406>] kunit_try_run_case+0x10e/0x290
 [<00000003e51b7dfa>] kunit_generic_run_threadfn_adapter+0x62/0xb8
 [<00000003e3ce30f8>] kthread+0x2d0/0x398
 [<00000003e3b96afa>] __ret_from_fork+0x8a/0xe8
 [<00000003e60ccada>] ret_from_fork+0xa/0x40

The reason for this error report is that kprobes handling code failed
to restore irqs.

The problem is that when kprobe is triggered from another kprobe
post_handler current sequence of enable_singlestep / disable_singlestep
is the following:
enable_singlestep  <- original kprobe (saves kprobe_saved_imask)
enable_singlestep  <- kprobe triggered from post_handler (clobbers kprobe_saved_imask)
disable_singlestep <- kprobe triggered from post_handler (restores kprobe_saved_imask)
disable_singlestep <- original kprobe (restores wrong clobbered kprobe_saved_imask)

There is just one kprobe_ctlblk per cpu and both calls saves and
loads irq mask to kprobe_saved_imask. To fix the problem simply move
resume_execution (which calls disable_singlestep) before calling
post_handler. This also fixes the problem that post_handler is called
with pt_regs which were not yet adjusted after single-stepping.

Cc: stable@vger.kernel.org
Fixes: 4ba069b8 ("[S390] add kprobes support.")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>