btrfs: save irq flags when looking up an ordered extent

A following patch will add another caller of
btrfs_lookup_ordered_extent(), but from a bio's endio context.

btrfs_lookup_ordered_extent() uses spin_lock_irq() which unconditionally
disables interrupts. Change this to spin_lock_irqsave() so interrupts
aren't disabled and re-enabled unconditionally.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ordered-data.c

@@ -767,9 +767,10 @@ struct btrfs_ordered_extent *btrfs_lookup_ordered_extent(struct btrfs_inode *ino
 	struct btrfs_ordered_inode_tree *tree;
 	struct rb_node *node;
 	struct btrfs_ordered_extent *entry = NULL;
+	unsigned long flags;
 
 	tree = &inode->ordered_tree;
-	spin_lock_irq(&tree->lock);
+	spin_lock_irqsave(&tree->lock, flags);
 	node = tree_search(tree, file_offset);
 	if (!node)
 		goto out;
@@ -780,7 +781,7 @@ struct btrfs_ordered_extent *btrfs_lookup_ordered_extent(struct btrfs_inode *ino
 	if (entry)
 		refcount_inc(&entry->refs);
 out:
-	spin_unlock_irq(&tree->lock);
+	spin_unlock_irqrestore(&tree->lock, flags);
 	return entry;
 }