x86,kgdb: fix debugger hw breakpoint test regression in 2.6.35

HW breakpoints events stopped working correctly with kgdb as a result
of commit: 018cbffe (Merge commit
'v2.6.33' into perf/core), later commit:
ba773f7c (x86,kgdb: Fix hw breakpoint
regression) allowed breakpoints to propagate to the debugger core but
did not completely address the original regression in functionality
found in 2.6.35.

When the DR_STEP flag is set in dr6 along with any of the DR_TRAP
bits, the kgdb exception handler will enter once from the
hw_breakpoint API call back and again from the die notifier for
do_debug(), which causes the debugger to stop twice and also for the
kgdb regression tests to fail running under kvm with:

echo V2I1 > /sys/module/kgdbts/parameters/kgdbts

To address the problem, the kgdb overflow handler needs to implement
the same logic as the ptrace overflow handler call back with respect
to updating the virtual copy of dr6.  This will allow the kgdb
do_debug() die notifier to properly handle the exception and the
attached debugger, or kgdb test suite, will only receive a single
notification.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: x86@kernel.org