Fix adjustments made to MRU and MFU in arc_adjust

As far as I can tell, the calculations used to determine the adjustments
made to the arc's lists are misplaced. From my understanding of the
original paper describing the arc algorithm, the following statements
should be upheld:

    * arc_size is kept below arc_c
    * the mru list size is kept below arc_p
    * the mfu list size is kept below (arc_c - arc_p)
    * the sum of the mru list size, mru ghost list size, mfu list size,
      and mfu ghost list size should be kept below (2 * arc_c)

Unfortunately, I don't think the code does this in it's current state;
and this change is an attempt to correct this.

Signed-off-by: Prakash Surya <surya1@llnl.gov>