Btrfs: fix page leakage

Alloc_dummy_extent_buffer will not free the first page in the eb array if we
fail to allocate a page, fix this.  Thanks,

Reported-by: David Sterba <dave@jikos.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/extent_io.c

@@ -4104,7 +4104,7 @@ struct extent_buffer *alloc_dummy_extent_buffer(u64 start, unsigned long len)
 
 	return eb;
 err:
-	for (i--; i > 0; i--)
+	for (i--; i >= 0; i--)
 		__free_page(eb->pages[i]);
 	__free_extent_buffer(eb);
 	return NULL;