powerpc/64s: fix handling of non-synchronous machine checks

A synchronous machine check is an exception raised by the attempt to
execute the current instruction. If the error can't be corrected, it
can make sense to SIGBUS the currently running process.

In other cases, the error condition is not related to the current
instruction, so killing the current process is not the right thing to
do.

Today, all machine checks are MCE_SEV_ERROR_SYNC, so this has no
practical change. It will be used to handle POWER9 asynchronous
machine checks.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>