set autotrim default to 'off' everywhere

As it turns out having autotrim default to 'on' on FreeBSD never really
worked due to mess with defines where userland and kernel module were
getting different default values (userland was defaulting to 'off',
module was thinking it's 'on').

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Yuri Pankov <yuripv@FreeBSD.org>
Closes #15079