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/disk-io.c b/fs/btrfs/disk-io.c
index adda739..3e18175 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -857,7 +857,7 @@
 	struct inode *btree_inode = root->fs_info->btree_inode;
 	if (btrfs_header_generation(buf) ==
 	    root->fs_info->running_transaction->transid) {
-		WARN_ON(!btrfs_tree_locked(buf));
+		btrfs_assert_tree_locked(buf);
 
 		/* ugh, clear_extent_buffer_dirty can be expensive */
 		btrfs_set_lock_blocking(buf);
@@ -2361,7 +2361,7 @@
 
 	btrfs_set_lock_blocking(buf);
 
-	WARN_ON(!btrfs_tree_locked(buf));
+	btrfs_assert_tree_locked(buf);
 	if (transid != root->fs_info->generation) {
 		printk(KERN_CRIT "btrfs transid mismatch buffer %llu, "
 		       "found %llu running %llu\n",