btrfs: rename btrfs_discard workqueue to btrfs-discard

We use the "btrfs-" prefix for our workqueues, the discard has
underscore instead of dash, so unify it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>