MFV r305560: 7278 tuning zfs_arc_max does not impact arc_c_min

MFV r305560: 7278 tuning zfs_arc_max does not impact arc_c_min

When changing zfs_arc_max (e.g. as zdb does), it may be set to less
than the default arc_c_min. arc_c_min should decrease to not be more than
arc_c_max, but it doesn't; therefore tuning of arc_c_max is ineffective.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Author: Matthew Ahrens <mahrens@delphix.com>

openzfs/openzfs@608764beadaf4bb71c5d8fe1818e8392ac66a61b