nr_pdcp_entity_recv_sdu() processes PDCP SDUs, and forwards the PDU at the
end of the function, while having the PDCP lock. This can lead to
deadlocks, e.g., when another thread goes through RLC and tries to
access the PDCP while the PDCP tries to lock the RLC.

This commit changes the logic: instead of having recv_sdu() forward the
PDU directly, it fills a buffer with the PDU (thus the name change to
process_sdu()), and returns. We then unlock the PDCP, before forwarding
the PDU, thus eliminating potential deadlocks.