arc_meta_limit should be updated when arc_max is changed.

When arc_max is increased, arc_meta_limit will not be updated to 3/4
of the new arc_c_max value. This was done originally to preserve any
existing maximum value. This turned out to be counter intuitive to
users and this fix changes that behavior. If zfs_arc_meta_limit is
non-default, it will be picked up later in the ARC tuning function.

Signed-off-by: Gaurav Kumar <gaurav.kumar@nutanix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4893