Propagate extent_bytes change to autotrim thread

The autotrim thread only reads zfs_trim_extent_bytes_min and
zfs_trim_extent_bytes_max variable only on thread start. We
should check for parameter changes during thread execution to
allow parameter changes take effect without needing to disable
then restart the autotrim.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Václav Skála <skala@vshosting.cz>
Closes #14077