arm64: fix for bad_mode() handler to always result in panic

The bad_mode() handler is called if we encounter an uunknown exception,
with the expectation that the subsequent call to panic() will halt the
system. Unfortunately, if the exception calling bad_mode() is taken from
EL0, then the call to die() can end up killing the current user task and
calling schedule() instead of falling through to panic().

Remove the die() call altogether, since we really want to bring down the
machine in this "impossible" case.

Signed-off-by: Hari Vyas <hari.vyas@broadcom.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>