x86/kgdb: Return 0 from kgdb_arch_set_breakpoint()

err must be nonzero in order to reach text_poke(), which caused kgdb to
fail to set breakpoints:

  (gdb) break __x64_sys_sync
  Breakpoint 1 at 0xffffffff81288910: file ../fs/sync.c, line 124.
  (gdb) c
  Continuing.
  Warning:
  Cannot insert breakpoint 1.
  Cannot access memory at address 0xffffffff81288910

  Command aborted.

Fixes: 86a22057 ("x86/kgdb: Avoid redundant comparison of patched code")
Signed-off-by: Matt Mullins <mmullins@fb.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nadav Amit <namit@vmware.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190531194755.6320-1-mmullins@fb.com