sh: stacktrace: Properly terminate the trace entry buffer.

sh: stacktrace: Properly terminate the trace entry buffer.

This inserts a ULONG_MAX entry at the end of the valid entries in the
stack trace buffer so the default code doesn't need to scan to the end of
available slots. This also makes the trace buffer termination behaviour
consistent with the other architectures.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>