net: netconsole: fix wrong warning

In the Linux kernel, the following vulnerability has been resolved:

net: netconsole: fix wrong warning

A warning is triggered when there is insufficient space in the buffer

for userdata. However, this is not an issue since userdata will be sent

in the next iteration.

Current warning message:

------------[ cut here ]------------

WARNING: CPU: 13 PID: 3013042 at drivers/net/netconsole.c:1122 write_ext_msg+0x3b6/0x3d0

? write_ext_msg+0x3b6/0x3d0

console_flush_all+0x1e9/0x330

The code incorrectly issues a warning when this_chunk is zero, which is

a valid scenario. The warning should only be triggered when this_chunk

is negative.