kill-the-bkl/reiserfs: fix reiserfs lock to cpu_add_remove_lock dependency

While creating the reiserfs workqueue during the journal
initialization, we are holding the reiserfs lock, but
create_workqueue() also holds the cpu_add_remove_lock, creating
then the following dependency:

- reiserfs lock -> cpu_add_remove_lock

But we also have the following existing dependencies:

- mm->mmap_sem -> reiserfs lock
- cpu_add_remove_lock -> cpu_hotplug.lock -> slub_lock -> sysfs_mutex

The merged dependency chain then becomes:

- mm->mmap_sem -> reiserfs lock -> cpu_add_remove_lock ->
	cpu_hotplug.lock -> slub_lock -> sysfs_mutex

But when we fill a dir entry in sysfs_readir(), we are holding the
sysfs_mutex and we also might fault while copying the directory entry
to the user, leading to the following dependency:

- sysfs_mutex -> mm->mmap_sem

The end result is then a lock inversion between sysfs_mutex and
mm->mmap_sem, as reported in the following lockdep warning:

[ INFO: possible circular locking dependency det...