usb: core: kcov: collect coverage from usb complete callback

This patch adds kcov_remote_start/stop() callbacks around the urb
complete() callback that is executed in softirq context when dummy_hcd is
in use.  As the result, kcov can be used to collect coverage from those
callbacks, which is used to facilitate coverage-guided fuzzing with
syzkaller.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Marco Elver <elver@google.com>
Link: http://lkml.kernel.org/r/4520671eeb604adbc2432c248b0c07fbaa5519ef.1585233617.git.andreyknvl@google.com
Link: http://lkml.kernel.org/r/2821d497ac1cdc0efb5e00df30271e4a67fc8009.1584655448.git.andreyknvl@google.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>