dm raid1: fix do_failures

Missing braces.  Commit 1f965b19 (dm raid1: separate region_hash interface
part1) broke it.

Signed-off-by: Ilpo Jarvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Heinz Mauelshagen <hjm@redhat.com>