fs: make dedupe work again after a really unfortunate build fix

In commit 14ce81c08 "fs: get rid of silly base class that causes build
failures now" I neglected to set the dest_count field in the ioctl
arg structure, so bees master hasn't been deduping anything for about
three weeks.

I'd put a THROW_CHECK in here to catch this kind of bug in the future,
but it would be placed at exactly the point where this fix is.

Fixes: 14ce81c08
Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fs.cc

@@ -143,6 +143,7 @@ namespace crucible {
 		ByteVector ioctl_arg( (btrfs_ioctl_same_args) {
 			.logical_offset = m_logical_offset,
 			.length = m_length,
+			.dest_count = ranged_cast<decltype(btrfs_ioctl_same_args::dest_count)>(m_info.size()),
 		}, buf_size);
 		btrfs_ioctl_same_args *const ioctl_ptr = ioctl_arg.get<btrfs_ioctl_same_args>();
 		size_t count = 0;