A bad page fault is when the HW signals an error such as a bad
copy/paste, an AMO error, or some other type of error that will
not be fixed by updating the PTE.

Use a helper page_fault_is_bad() to check for bad page faults thus
removing the per-processor family open-coding in __do_page_fault()
and trigger a SIGBUS rather than a SIGSEGV which is more appropriate.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>