lib: fix btrfs_data_container pointer casts for 32-bit userspace on 64-bit kernels

Apparently reinterpret_cast<uint64_t> sign-extends 32-bit pointers.
This is OK when running on a 32-bit kernel that will truncate the pointer
to 32 bits, but when running on a 64-bit kernel, the extra bits are
interpreted as part of the (now very invalid) address.

Use <uintptr_t> instead, which is unsigned, integer, and the same word
size as the arch's pointer type.  Ordinary numeric conversion can take
it from there, filling the rest of the word with zeros.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fs.cc

@@ -333,7 +333,7 @@ namespace crucible {
 		btrfs_ioctl_logical_ino_args args = (btrfs_ioctl_logical_ino_args) {
 			.logical = m_logical,
 			.size = m_container_size,
-			.inodes = reinterpret_cast<uint64_t>(m_container.prepare(m_container_size)),
+			.inodes = reinterpret_cast<uintptr_t>(m_container.prepare(m_container_size)),
 		};
 		// We are still supporting building with old headers that don't have .flags yet
 		*(&args.reserved[0] + 3) = m_flags;
@@ -416,7 +416,7 @@ namespace crucible {
 	{
 		btrfs_ioctl_ino_path_args *p = static_cast<btrfs_ioctl_ino_path_args *>(this);
 		BtrfsDataContainer container(m_container_size);
-		fspath = reinterpret_cast<uint64_t>(container.prepare(m_container_size));
+		fspath = reinterpret_cast<uintptr_t>(container.prepare(m_container_size));
 		size = container.get_size();
 
 		m_paths.clear();