btrfs: use ASSERT to report logical error in cow_file_range()

Use ASSERT to report logical error in cow_file_range(), also move it a
bit closer to when the num_bytes is derived.

The extent start could be (u64)-1 in some cases, the assert should catch
that we do not accidentally pass it to cow_file_range.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -978,6 +978,7 @@ static noinline int cow_file_range(struct inode *inode,
 
 	num_bytes = ALIGN(end - start + 1, blocksize);
 	num_bytes = max(blocksize,  num_bytes);
+	ASSERT(num_bytes <= btrfs_super_total_bytes(fs_info->super_copy));
 
 	inode_should_defrag(BTRFS_I(inode), start, end, num_bytes, SZ_64K);
 
@@ -1008,8 +1009,6 @@ static noinline int cow_file_range(struct inode *inode,
 		}
 	}
 
-	BUG_ON(num_bytes > btrfs_super_total_bytes(fs_info->super_copy));
-
 	alloc_hint = get_extent_allocation_hint(inode, start, num_bytes);
 	btrfs_drop_extent_cache(BTRFS_I(inode), start,
 			start + num_bytes - 1, 0);