net: buf: Simplify fragment handling

This patch reworks how fragments are handled in the net_buf
infrastructure.

In particular, it removes the union around the node and frags members
in the main net_buf structure. This is done so that both can be used at
the same time, at a cost of 4 bytes per net_buf instance.
This implies that the layout of net_buf instances changes whenever
being inserted into a queue (fifo or lifo) or a linked list (slist).

Until now, this is what happened when enqueueing a net_buf with frags
in a queue or linked list:

1.1 Before enqueueing:

 +--------+      +--------+      +--------+
 |#1  node|\     |#2  node|\     |#3  node|\
 |        | \    |        | \    |        | \
 | frags  |------| frags  |------| frags  |------NULL
 +--------+      +--------+      +--------+

net_buf #1 has 2 fragments, net_bufs #2 and #3. Both the node and frags
pointers (they are the same, since they are unioned) point to the next
fragment.

1.2 After enqueueing:

 +--------+     +--------+     +--------+     +--------+     +--------+
 |q/slist |-----|#1  node|-----|#2  node|-----|#3  node|-----|q/slist |
 |node    |     | *flag  | /   | *flag  | /   |        | /   |node    |
 |        |     | frags  |/    | frags  |/    | frags  |/    |        |
 +--------+     +--------+     +--------+     +--------+     +--------+

When enqueing a net_buf (in this case #1) that contains fragments, the
current net_buf implementation actually enqueues all the fragments (in
this case #2 and #3) as actual queue/slist items, since node and frags
are one and the same in memory. This makes the enqueuing operation
expensive and it makes it impossible to atomically dequeue. The `*flag`
notation here means that the `flags` member has been set to
`NET_BUF_FRAGS` in order to be able to reconstruct the frags pointers
when dequeuing.

After this patch, the layout changes considerably:

2.1 Before enqueueing:

 +--------+       +--------+       +--------+
 |#1  node|--NULL |#2  node|--NULL |#3  node|--NULL
 |        |       |        |       |        |
 | frags  |-------| frags  |-------| frags  |------NULL
 +--------+       +--------+       +--------+

This is very similar to 1.1, except that now node and frags are
different pointers, so node is just set to NULL.

2.2 After enqueueing:

 +--------+      +--------+      +--------+
 |q/slist |------|#1  node|------|q/slist |
 |node    |      |        |      |node    |
 |        |      | frags  |      |        |
 +--------+      +--------+      +--------+
                     |           +--------+       +--------+
                     |           |#2  node|--NULL |#3  node|--NULL
                     |           |        |       |        |
                     +-----------| frags  |-------| frags  |------NULL
                                 +--------+       +--------+

When enqueuing net_buf #1, now we only enqueue that very item, instead
of enqueing the frags as well, since now node and frags are separate
pointers. This simplifies the operation and makes it atomic.

Resolves #52718.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
(cherry picked from commit 3d306c18)