Use bitvec_set_*() directly without external write pointer tracking to
simplify the code. This is part of IA Rest Octets (3GPP TS 44.018
§10.5.2.16) which is the last part of the message so it should not
interfere with the rest of encoding functions.
The difference in the expected test output is due to proper handling of
TAI which should not be transmitted for SBA according to the Note in
Table 10.5.2.16.1 in 3GPP TS 44.018.
The change was manually tested against real mobile phone using options
'gprs mode gprs' in osmo-bsc.cfg and 'two-phase-access' in osmo-pcu.cfg
to make sure appropriate code path is actually triggered.
That's partially based on reverted commit 93d947f5e8.
Change-Id: I97d53c27c1ca9e032d431b3aa7f915027d63ddc0
Related: OS#3014
3eb47363ad