dm raid: use read_disk_sb() throughout

For consistency, call read_disk_sb() from
attempt_restore_of_faulty_devices() instead
of calling sync_page_io() directly.

Explicitly set device to faulty on superblock read error.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>