btrfs: reinitialize scrub workers

Scrub starts the workers each time a scrub starts and stops them after it
finished. This patch adds an initialization for the workers before each
start, otherwise the workers behave strangely.

Signed-off-by: Arne Jansen <sensille@gmx.net>

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/scrub.c
Simple merge