zdtm/sigpending: check only relevant part of siginfo

Recently, a kernel behaviour how siginfo is copied into user-space was
changed:

4ce5f9c9e754 ("signal: Use a smaller struct siginfo in the kernel")

According to the rt_sigqueueinfo man page, the caller should set
si_code, si_pid, si_uid, si_value and there is no guarantee about other
fields.

Signed-off-by: Andrei Vagin <avagin@gmail.com>