linux: Fix i686 with gcc6

On __convert_scm_timestamps GCC 6 issues an warning that tvts[0]/tvts[1]
maybe be used uninitialized, however it would be used if type is set to a
value different than 0 (done by either COMPAT_SO_TIMESTAMP_OLD or
COMPAT_SO_TIMESTAMPNS_OLD) which will fallthrough to 'common' label.

It does not show with gcc 7 or more recent versions.

Checked on i686-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>