seccomp: passthrough uretprobe systemcall without filtering

In the Linux kernel, the following vulnerability has been resolved:

seccomp: passthrough uretprobe systemcall without filtering

When attaching uretprobes to processes running inside docker, the attached

process is segfaulted when encountering the retprobe.

The reason is that now that uretprobe is a system call the default seccomp

filters in docker block it as they only allow a specific set of known

syscalls. This is true for other userspace applications which use seccomp

to control their syscall surface.

Since uretprobe is a "kernel implementation detail" system call which is

not used by userspace application code directly, it is impractical and

there's very little point in forcing all userspace applications to

explicitly allow it in order to avoid crashing tracked processes.

Pass this systemcall through seccomp without depending on configuration.

Note: uretprobe is currently only x86_64 and isn't expected to ever be

supported in i386.

[kees: minimized changes for easier backporting, tweaked commit log]