Fix arc_adjust_meta_balanced()

The type of "adjustmnt" was erroneously changed to unsigned when the compressed
ARC code was ported in d3c2ae1c0806b183a315e3d43cc8018cfdca79b5.

As a result of it being unsigned, the balanced metadata eviction logic
would evict all of the non-metadata.

Reviewed-by: Chris Severance <github.severach@spamgourmet.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: David Quigley <david.quigley@intel.com>
Signed-off-by: Tim Chase <tim@onlight.com>
Closes #5128
Closes #5129