powerpc/ptrace: Fix brk.len used uninitialised

With some CONFIGS it's possible that in ppc_set_hwdebug, brk.len is
uninitialised before being used.  It has been reported that GCC 4.2 will
produce the following error in this case:

  arch/powerpc/kernel/ptrace.c:1479: warning: 'brk.len' is used uninitialized in this function
  arch/powerpc/kernel/ptrace.c:1381: note: 'brk.len' was declared here

This patch corrects this.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reported-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>