ranap_rab_ass_req_encode() forms a msgb, then copies the data to a
buffer provided by the caller. Instead, just return the msgb. This
removes one unnecessary memcpy() and simplifies some code.
In ranap_rab_ass_test.c, actually ensure the correct size of the
returned data. See also the fix of expected test data in patch
Ifb98a52e56db1227a834c0d7b7a260314d9f547e
Related: SYS#5895
Change-Id: I85e715326e1d8f4f301f82f78da109f1a7a92f30
223aeda282