In NSA, the RA process contains the RNTI of the UE to add. The previous
implementation would not remove the RNTI in NSA/for CFRA, which is
unclear (why should it stay? RA is over). Also, we did not clear all
contents, which masked a bug, as we were reusing an RA process for a
subsequent attach, leading to a segfault on reattach. That is fixed in
this commit.

Also, we were using the RA structure after having cleared it in
_nr_rx_sdu(), which semantically makes no sense.

To harmonize handling of the preambles: completely reset the RA
processes inside nr_clear_ra_proc(). For SA, also add the initialization
of allowed preambles.

Finally, this commit fixes a memory leak: we always (also in SA) fill a
list of preambles in a dynamically allocated preamble list. Since the
total list is only up to 64 preambles of 1B each, put it directly into
the struct instead of dynamically allocating it.