seccomp: add a selftest for get_metadata

[ Upstream commit d057dc4e35e16050befa3dda943876dab39cbf80 ]

Let's test that we get the flags correctly, and that we preserve the filter
index across the ptrace(PTRACE_SECCOMP_GET_METADATA) correctly.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>