seccomp: kill the seccomp_t typedef

Replaces the seccomp_t typedef with struct seccomp to match modern
kernel style.

Signed-off-by: Will Drewry <wad@chromium.org>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Eric Paris <eparis@redhat.com>

v18: rebase
...
v14: rebase/nochanges
v13: rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: rebase on to linux-next
v8-v11: no changes
v7: struct seccomp_struct -> struct seccomp
v6: original inclusion in this series.