BACKPORT: seccomp: fix build warnings when there is no CONFIG_SECCOMP_FILTER

BACKPORT: seccomp: fix build warnings when there is no CONFIG_SECCOMP_FILTER

If both audit and seccomp filter support are disabled, 'ret' is marked
as unused.

If just seccomp filter support is disabled, data and skip are considered
unused.

This change fixes those build warnings.

Change-Id: I2ec787dc6703cf5055e8e19b0800c2e80d0e73f4
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>
BUG=chromium-os:27878
TEST=none. Part of a much larger stack that really should merge together.

Change-Id: Id501094847dfb3bd67f1d3e91996939e495ab712
Reviewed-on: https://gerrit.chromium.org/gerrit/21373
Reviewed-by: Will Drewry <wad@chromium.org>
Tested-by: Will Drewry <wad@chromium.org>