powerpc: Use correct procedure call standard for getrandom vDSO call (bug 32440)

powerpc: Use correct procedure call standard for getrandom vDSO call (bug 32440)

A plain indirect function call does not work on POWER because
success and failure are signaled through a flag register, and
not via the usual Linux negative return value convention.

This has potential security impact, in two ways: the return value
could be out of bounds (EAGAIN is 11 on powerpc6le), and no
random bytes have been written despite the non-error return value.

Fixes commit 461cab1de747f3842f27a5d24977d78d561d45f9 ("linux: Add
support for getrandom vDSO").

Reported-by: Ján Stanček <jstancek@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>