powerpc/ptrace: Enable hardware breakpoint upon re-registering

On powerpc, ptrace will disable hardware breakpoint request once the
breakpoint is hit. It is the responsibility of the caller to set it
again. However, when the caller sets the hardware breakpoint again
using ptrace(PTRACE_SET_DEBUGREG, child_pid, 0, addr), the hardware
breakpoint is not enabled.

While gdb's approach is to unregister and re-register the hardware
breakpoint every time the breakpoint is hit - which is working fine,
this could affect other programs trying to re-register hardware
breakpoint without unregistering.

This patch enables hardware breakpoint if the caller is re-registering.

Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>