This function should only return "linux" on Linux.
Move the kernel part of the function out of common code.
Fix the tests for FreeBSD.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#10079