x86,kgdb: Always initialize the hw breakpoint attribute

It is required to call hw_breakpoint_init() on an attr before using it
in any other calls.  This fixes the problem where kgdb will sometimes
fail to initialize on x86_64.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: 2.6.33 <stable@kernel.org>
LKML-Reference: <1269975907-27602-1-git-send-email-jason.wessel@windriver.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>