kcov: support compat processes

Support compat processes in KCOV by providing compat_ioctl callback.
Compat mode uses the same ioctl callback: we have 2 commands that do not
use the argument and 1 that already checks that the arg does not overflow
INT_MAX.  This allows to use KCOV-guided fuzzing in compat processes.

Link: http://lkml.kernel.org/r/20170823100553.55812-1-dvyukov@google.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kcov.c

@@ -270,6 +270,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
 static const struct file_operations kcov_fops = {
 	.open		= kcov_open,
 	.unlocked_ioctl	= kcov_ioctl,
+	.compat_ioctl	= kcov_ioctl,
 	.mmap		= kcov_mmap,
 	.release        = kcov_close,
 };