When the UE receives several PDCP packets in one MAC PDU, if the
first is SecurityModeCommand, the following will be ciphered/integrity
protected. The RRC has to process SecurityModeCommand before PDCP
processes the following packets. Before this commit, PDCP will
not wait for RRC to finish and process the following messages, which
won't make any sense since they are ciphered but ciphering is not
configured yet.

So let's introduce some synchronous commmunication between PDCP
and RRC for SRBs in the UE. PDCP sends the data to RRC and waits
for a return message from RRC to continue its job.

The code is not super satisfying. nr_pdcp_entity_recv_pdu() may queue
the message and not send it to RRC immediately, so do_pdcp_data_ind()
may wait for a long time and maybe forever, I did  not analyze things
properly here. (The queuing is easy to detect, we could AssertFatal()
if that happens. Then find a proper solution if the case happens for
real and turns out being super annoying.)

Just an attempt, let's see.