bcachefs: Fix 'hung task' messages in btree node scan

bcachefs: Fix 'hung task' messages in btree node scan

btree node scan has to wait on kthread workers that scan each device,
potentially for awhile.

We would like this to be interruptible, but we may need a different
mechanism than signals for that - we've had bugs in the past where
mounts were failing due to checking for signals, and no explanation on
where they came from.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>