btrfs: Remove WARN_ON in btrfs_alloc_delalloc_work

It can never trigger since before calling alloc_delalloc_work we have
called igrab in start_delalloc_inodes.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>