x86, tracehook: clean up implementation of syscall_get_error()

The x86-tracehook code now contains this line in syscall_get_error():

	return error >= -4095L ? error : 0;

Hard-wiring a constant is not nice. Let's use the IS_ERR_VALUE macro
from linux/err.h instead.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Cc: utrace-devel@redhat.com
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>