net: netconsole: Correct mismatched return types

netconsole incorrectly mixes int and ssize_t types by using int for
return variables in functions that should return ssize_t.

This is fixed by updating the return variables to the appropriate
ssize_t type, ensuring consistency across the function definitions.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>