kcov: properly check for softirq context

When collecting coverage from softirqs, KCOV uses in_serving_softirq() to
check whether the code is running in the softirq context.  Unfortunately,
in_serving_softirq() is > 0 even when the code is running in the hardirq
or NMI context for hardirqs and NMIs that happened during a softirq.

As a result, if a softirq handler contains a remote coverage collection
section and a hardirq with another remote coverage collection section
happens during handling the softirq, KCOV incorrectly detects a nested
softirq coverate collection section and prints a WARNING, as reported by
syzbot.

This issue was exposed by commit a7f3813e589f ("usb: gadget: dummy_hcd:
Switch to hrtimer transfer scheduler"), which switched dummy_hcd to using
hrtimer and made the timer's callback be executed in the hardirq context.

Change the related checks in KCOV to account for this behavior of
in_serving_softirq() and make KCOV ignore remote coverage collection
sections in the hardirq and NMI contexts.

This prevents the WARNING printed by syzbot but does not fix the inability
of KCOV to collect coverage from the __usb_hcd_giveback_urb when dummy_hcd
is in use (caused by a7f3813e589f); a separate patch is required for that.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts")
Signed-off-by: Andrey Konovalov <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=2388cdaeb6b10f0c13ac
Acked-by: Marco Elver <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Aleksandr Nogikh <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Marcello Sylvester Bauer <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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