bluetooth: mesh: Remove illegal use of NET_BUF_FRAG in friend.c

This commit removes illegal use of NET_BUF_FRAG in friend.c, which is an
internal flag.

Now `struct bt_mesh_friend_seg` keeps pointer to a first received
segment of a segmented message. The rest segments are added as fragments
using net_buf API. Friend Queue keeps only head of the fragments.
When one segment (currently head of fragments) is removed from Friend
Queue, the next segment is added to the queue. Head has always 2
references: one when allocated, another one when added as fragments
head.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
(cherry picked from commit 5d059117)