examples/link-cp: avoid misaligned struct io_data access

queue_rw_pair() placed struct io_data at ptr + size, so any block size
not a multiple of 8 (the partial tail block of a copy) accessed the
io_data members through a misaligned pointer, which is undefined
behavior and faults under -fsanitize=alignment. Allocate the struct
first and point iov_base at data + 1, matching io_uring-cp.c, and free
data directly.

Signed-off-by: rootvector2 <dxbnaveed.k@gmail.com>