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.h b/fs/btrfs/locking.h
index 6bb0afb..6c4ce45 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -21,11 +21,11 @@
 
 int btrfs_tree_lock(struct extent_buffer *eb);
 int btrfs_tree_unlock(struct extent_buffer *eb);
-int btrfs_tree_locked(struct extent_buffer *eb);
 
 int btrfs_try_tree_lock(struct extent_buffer *eb);
 int btrfs_try_spin_lock(struct extent_buffer *eb);
 
 void btrfs_set_lock_blocking(struct extent_buffer *eb);
 void btrfs_clear_lock_blocking(struct extent_buffer *eb);
+void btrfs_assert_tree_locked(struct extent_buffer *eb);
 #endif