x86_64/traps: Fix always true condition

We should be checking IS_ERR() here.  PTR_ERR() is always true.

Fixes: fe3d197f ('x86, mpx: On-demand kernel allocation of
bounds tables')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Link: http://lkml.kernel.org/r/20141125172114.GA24535@mwanda


Signed-off-by: Ingo Molnar <mingo@kernel.org>