Illumos 6033 - arc_adjust() should search MFU lists

Illumos 6033 - arc_adjust() should search MFU lists

6033 arc_adjust() should search MFU lists for oldest
buffer when adjusting MFU size
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Xin Li <delphij@delphij.net>
Reviewed by: Prakash Surya <me@prakashsurya.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

References:
  https://www.illumos.org/issues/6033
  https://github.com/illumos/illumos-gate/commit/31c46cf

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3545