x86: Ignore trap bits on single step exceptions

When a single step exception fires, the trap bits, used to
signal hardware breakpoints, are in a random state.

These trap bits might be set if another exception will follow,
like a breakpoint in the next instruction, or a watchpoint in the
previous one. Or there can be any junk there.

So if we handle these trap bits during the single step exception,
we are going to handle an exception twice, or we are going to
handle junk.

Just ignore them in this case.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=21332



Reported-by: Michael Stefaniuc <mstefani@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Maciej Rutecki <maciej.rutecki@gmail.com>
Cc: Alexandre Julliard <julliard@winehq.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: All since 2.6.33.x <stable@kernel.org>