syzkaller: include code snippets into WARNING reports

[a-nogikh]

From LKML:

> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 20504 at mm/vma.c:734 vma_merge_existing_range+0x1145/0x16f0 mm/vma.c:734

It'd be nice if syzbot could actually print the code that generates the
warning :) a nice-to-have perhaps.

[tarasmadan]

It currently requires to:

1. Click on the link in syzbot e-mail to open dashboard page.
2. Scroll to the crash report.
3. Click on the WARNING mm/vma.c:734 link.

Current complexity looks reasonable to me...

[tarasmadan]

@FlorentRevest wdyt?

[dvyukov]

It may be better for the kernel to do it. Replied on the email thread.