Btrfs: Fix mismerge in block header checks

I had incorrectly disabled the check for the block number being correct
in the header block.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 57fbf10..f6f7821 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -346,7 +346,7 @@
 	eb = alloc_extent_buffer(tree, start, len, page, GFP_NOFS);
 
 	found_start = btrfs_header_bytenr(eb);
-	if (0 && found_start != start) {
+	if (found_start != start) {
 		printk("bad tree block start %llu %llu\n",
 		       (unsigned long long)found_start,
 		       (unsigned long long)eb->start);