netconsole: consolidate send buffers into netconsole_target struct

Move the static buffers from send_msg_no_fragmentation() and
send_msg_fragmented() into the netconsole_target structure. This
simplifies the code by:
- Eliminating redundant static buffers
- Centralizing buffer management in the target structure
- Reducing memory usage by 1KB (one buffer instead of two)

The buffer in netconsole_target is protected by target_list_lock,
maintaining the same synchronization semantics as the original code.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>