m68k: Only force 030 bus error if PC not in exception table

In the Linux kernel, the following vulnerability has been resolved:

m68k: Only force 030 bus error if PC not in exception table

__get_kernel_nofault() does copy data in supervisor mode when

forcing a task backtrace log through /proc/sysrq_trigger.

This is expected cause a bus error exception on e.g. NULL

pointer dereferencing when logging a kernel task has no

workqueue associated. This bus error ought to be ignored.

Our 030 bus error handler is ill equipped to deal with this:

Whenever ssw indicates a kernel mode access on a data fault,

we don't even attempt to handle the fault and instead always

send a SEGV signal (or panic). As a result, the check

for exception handling at the fault PC (buried in

send_sig_fault() which gets called from do_page_fault()

eventually) is never used.

In contrast, both 040 and 060 access error handlers do not

care whether a fault happened on supervisor mode access,

and will call do_page_fault() on those, ultimately honoring

the exception table.

Add a check in bus_error030 to call do_page_fault() in case

we do have an entry for the fault PC in our exception table.

I had attempted a fix for this earlier in 2019 that did rely

on testing pagefault_disabled() (see link below) to achieve

the same thing, but this patch should be more generic.

Tested on 030 Atari Falcon.