Fix: signal: Distinguish between kernel_siginfo and siginfo (v4.20)

Fix: signal: Distinguish between kernel_siginfo and siginfo (v4.20)

See upstream commit :

  commit ae7795bc6187a15ec51cf258abae656a625f9980
  Author: Eric W. Biederman <ebiederm@xmission.com>
  Date:   Tue Sep 25 11:27:20 2018 +0200

    signal: Distinguish between kernel_siginfo and siginfo

    Linus recently observed that if we did not worry about the padding
    member in struct siginfo it is only about 48 bytes, and 48 bytes is
    much nicer than 128 bytes for allocating on the stack and copying
    around in the kernel.

    The obvious thing of only adding the padding when userspace is
    including siginfo.h won't work as there are sigframe definitions in
    the kernel that embed struct siginfo.

    So split siginfo in two; kernel_siginfo and siginfo.  Keeping the
    traditional name for the userspace definition.  While the version that
    is used internally to the kernel and ultimately will not be padded to
    128 bytes is called kernel_siginfo.

    The definition of struct kernel_siginfo I have put in include/signal_types.h

    A set of buildtime checks has been added to verify the two structures have
    the same field offsets.

    To make it easy to verify the change kernel_siginfo retains the same
    size as siginfo.  The reduction in size comes in a following change.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>