Btrfs: don't look at the extent buffer level 3 times in a row

We have a bit of debugging in btrfs_search_slot to make sure the level of the
cow block is the same as the original block we were cow'ing.  I don't think I've
ever seen this tripped, so kill it.  This saves us 2 kmap's per level in our
search.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/ctree.c

@@ -1672,9 +1672,6 @@ again:
 		}
 cow_done:
 		BUG_ON(!cow && ins_len);
-		if (level != btrfs_header_level(b))
-			WARN_ON(1);
-		level = btrfs_header_level(b);
 
 		p->nodes[level] = b;
 		if (!p->skip_locking)