btrfs: Remove unnecessary casts in btrfs_read_root_item

There is a messy cast here:
	min_t(int, len, (int)sizeof(*item)));

min_t() should normally cast to unsigned.  It's not possible for "len"
to be negative, but if it were then we definitely wouldn't want to pass
negatives to read_extent_buffer().  Also there is an extra cast.

This patch shouldn't affect runtime, it's just a clean up.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/root-tree.c b/fs/btrfs/root-tree.c
index 65bda06..0d2b957 100644
--- a/fs/btrfs/root-tree.c
+++ b/fs/btrfs/root-tree.c
@@ -21,12 +21,12 @@
 				struct btrfs_root_item *item)
 {
 	uuid_le uuid;
-	int len;
+	u32 len;
 	int need_reset = 0;
 
 	len = btrfs_item_size_nr(eb, slot);
 	read_extent_buffer(eb, item, btrfs_item_ptr_offset(eb, slot),
-			min_t(int, len, (int)sizeof(*item)));
+			   min_t(u32, len, sizeof(*item)));
 	if (len < sizeof(*item))
 		need_reset = 1;
 	if (!need_reset && btrfs_root_generation(item)