MFV r285025: 6033 arc_adjust() should search MFU lists for oldest buffer

MFV r285025: 6033 arc_adjust() should search MFU lists for oldest buffer
when adjusting MFU size.

illumos/illumos-gate@31c46cf23cd1cf4d66390a983dc5072d7d299ba2

https://www.illumos.org/issues/6033
  When we're looking for the list containing oldest buffer we never
  actually look at the MFU lists even when we try to evict from MFU.
  looks like a copy paste error, the fix is here:

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>
Author: Alek Pinchuk <alek@nexenta.com>
Obtained from:  illumos