btrfs: use correct type for workqueue flags

Through all the local wrappers to alloc_workqueue, __alloc_workqueue_key
takes an unsigned int.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/async-thread.c
Simple merge

diff --cc fs/btrfs/async-thread.h
Simple merge

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

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