recreate journal in mdadm

recreate journal in mdadm

This patch tries recreates missing/faulty journal in mdadm.

Example:

./mdadm --fail /dev/md1 /dev/sdb2
mdadm: set /dev/sdb2 faulty in /dev/md1

./mdadm --stop /dev/md1
mdadm: stopped /dev/md1

./mdadm -A --scan --force
mdadm: Journal is missing or stale, starting array read only.
mdadm: /dev/md/1 has been started with 15 drives.

./mdadm --add-journal /dev/md1 /dev/sdb2
mdadm: added /dev/sdb2

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>