zfs: set autotrim default to 'off'

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').

PR: 264234
Reviewed by: mav (zfs)
Differential Revision: https://reviews.freebsd.org/D41056