net: buf: fix data ref_count offset in generic_data_ref()

This patch fixes commit b70f92e5
("net: buf: keep memory alignment provided by k_heap_alloc and k_malloc").
One-line was overlooked in the above patch and may result in a
cloned net_buf using a data block that has already been freed.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>