MFC: sys/geom/mirror/g_mirror.c 1.90
sys/geom/raid3/g_raid3.c 1.74

Fix synchronization in gmirror and graid3 which I broken. Synchronization
request can still have bio_to set to sc_provider (this is READ part of a
synchronization request) and in this case g_{mirror,raid3}_sync() wasn't
called as it should be.

Approved by: re (mux)