netconsole: selftest: Add test for fragmented messages

Add a new selftest to verify netconsole's handling of messages that
exceed the packet size limit and require fragmentation. The test sends
messages with varying sizes and userdata, validating that:

1. Large messages are correctly fragmented and reassembled
2. Userdata fields are properly preserved across fragments
3. Messages work correctly with and without kernel release version
   appending

The test creates a networking environment using netdevsim, sends
messages through /dev/kmsg, and verifies the received fragments maintain
message integrity.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250203-netcons_frag_msgs-v1-1-5bc6bedf2ac0@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>