btrfs: free-space-cache: use unaligned helpers to access data

The free space inode stores the tracking data, checksums etc, using the
io_ctl structure and moving the pointers. The data are generally aligned
to at least 4 bytes (u32 for CRC) so it's not completely unaligned but
for clarity we should use the proper helpers whenever a struct is
initialized from io_ctl->cur pointer.

Signed-off-by: David Sterba <dsterba@suse.com>