Btrfs: fix spinlock assertions on UP systems

btrfs_tree_locked was being used to make sure a given extent_buffer was
properly locked in a few places.  But, it wasn't correct for UP compiled
kernels.

This switches it to using assert_spin_locked instead, and renames it to
btrfs_assert_tree_locked to better reflect how it was really being used.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 6b5966a..9abf81f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4418,13 +4418,13 @@
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
 
-	BUG_ON(!btrfs_tree_locked(parent));
+	btrfs_assert_tree_locked(parent);
 	parent_level = btrfs_header_level(parent);
 	extent_buffer_get(parent);
 	path->nodes[parent_level] = parent;
 	path->slots[parent_level] = btrfs_header_nritems(parent);
 
-	BUG_ON(!btrfs_tree_locked(node));
+	btrfs_assert_tree_locked(node);
 	level = btrfs_header_level(node);
 	extent_buffer_get(node);
 	path->nodes[level] = node;