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/locking.c b/fs/btrfs/locking.c
index 85506c4..47b0a88 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -220,8 +220,8 @@
 	return 0;
 }
 
-int btrfs_tree_locked(struct extent_buffer *eb)
+void btrfs_assert_tree_locked(struct extent_buffer *eb)
 {
-	return test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags) ||
-			spin_is_locked(&eb->lock);
+	if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+		assert_spin_locked(&eb->lock);
 }