Bluetooth: Mesh: Add extra flag to control seg msg in frnd queue

This commit fixes issue introduced in
5d059117.

Use extra flag stored in user data of net_buf to control segmented
messages in Friend Queue. The initial idea with using fragments didn't
work.

This fixes the following PTS tests:
- MESH/NODE/FRND/FN/BV-08-C
- MESH/NODE/FRND/FN/BV-19-C
- MESH/NODE/CFG/HBS/BV-05-C

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>