x86: Remove weird PTR_ERR() in do_debug

62edab905 changed the argument to notify_die() from dr6 to &dr6,
but weirdly, used PTR_ERR() to cast it to a long.  Since dr6 is
on the stack, this is an abuse of PTR_ERR().  Cast to long, as
per kernel standard.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: K.Prasad <prasad@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1371357768-4968-8-git-send-email-rusty@rustcorp.com.au
Signed-off-by: Ingo Molnar <mingo@kernel.org>