CHROMIUM: Fix bad forward port of seccomp PTRACE_O_MASK

CHROMIUM: Fix bad forward port of seccomp PTRACE_O_MASK

Make the mask 0xff and not 0x7f.  Without the change, it
is impossible to use PTRACE_O_TRACESECCOMP.

We should drop this and the original ported change and take
upstreams when we roll to 3.5+.

Signed-off-by: Will Drewry <wad@chromium.org>
TEST=seccomp_tests
BUG=none?

Change-Id: I4c7eb15d3b026203878e4f84ba23cbea28899c36
Reviewed-on: https://gerrit.chromium.org/gerrit/24649
Reviewed-by: Olof Johansson <olofj@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Will Drewry <wad@chromium.org>
Commit-Ready: Will Drewry <wad@chromium.org>