btrfs: mirror_num should be int, not u64

My previous patch introduced some u64 for failed_mirror variables, this one
makes it consistent again.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 47fdba7..a877b8b 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2285,8 +2285,8 @@
 				clean_io_failure(start, page);
 		}
 		if (!uptodate) {
-			u64 failed_mirror;
-			failed_mirror = (unsigned long)bio->bi_bdev;
+			int failed_mirror;
+			failed_mirror = (int)(unsigned long)bio->bi_bdev;
 			if (tree->ops && tree->ops->readpage_io_failed_hook)
 				ret = tree->ops->readpage_io_failed_hook(
 						bio, page, start, end,