The code is forced to use nea2, no matter what the UE supports.

After 2^18 PDCP packets, it will fail to work (we don't use HFN yet).

These limitations will be fixed in later commits.

The existing security function was not reused, because it does too
much memory allocation and initializes the security context at each
ciphering. So here comes nr_pdcp_security_nea2_cipher(). And also
the ciphering is done inplace. To be changed if necessary.