Fix zfs_arc_max minimum tuning

When setting zfs_arc_max its minimum value is allowed
to be 64 MiB. There was an off-by-1 error which can matter
on tiny systems.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Zubrzycki <github@mid-earth.net>
Closes #7417