dm: fix some sparse warnings and whitespace in dax methods

Eliminate these sparse warnings:
drivers/md/dm.c:1062:9: warning: context imbalance in 'dm_dax_direct_access' - unexpected unlock
drivers/md/dm.c:1086:9: warning: context imbalance in 'dm_dax_copy_from_iter' - unexpected unlock
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1021,6 +1021,7 @@ EXPORT_SYMBOL_GPL(dm_set_target_max_io_len);
 
 static struct dm_target *dm_dax_get_live_target(struct mapped_device *md,
 						sector_t sector, int *srcu_idx)
+	__acquires(md->io_barrier)
 {
 	struct dm_table *map;
 	struct dm_target *ti;