Btrfs: make sure we're not using obsolete code in btrfs_get_extent

There's code in btrfs_get_extent that should never be used. This patch turns
a WARN_ON(1) into a BUG(), hoping we can remove the transaction code from
btrfs_get_extent soon.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/inode.c

@@ -5022,7 +5022,7 @@ again:
 			}
 			flush_dcache_page(page);
 		} else if (create && PageUptodate(page)) {
-			WARN_ON(1);
+			BUG();
 			if (!trans) {
 				kunmap(page);
 				free_extent_map(em);