tests: update futex test to accommodate the recent kernel change

tests: update futex test to accommodate the recent kernel change

Kernel commit v4.14-rc1~162^2~8 refactored futex implementation and
introduced checks for invalid shift sizes.  Accommodate this change
by allowing EINVAL in the corresponding futex checks.

* tests/futex.c (CHECK_FUTEX_GENERIC): Reset errno before the syscall.
(main) <wake_ops>: Add err2 field, describe err/err2 fields.
Add EINVAL as a possible errno to the checks that contain invalid
shift values.
Update return value check so it checks that values are strictly
as expected.

Closes: https://github.com/strace/strace/pull/16