MFC r319750: MFV r319741: 8156 dbuf_evict_notify() does not need dbuf_evict_lock

illumos/illumos-gate@dbfd9f930004c390a2ce2cf850c71b4f880eef9c
https://github.com/illumos/illumos-gate/commit/dbfd9f930004c390a2ce2cf850c71b4f880eef9c

https://www.illumos.org/issues/8156

dbuf_evict_notify() holds the dbuf_evict_lock while checking if it should do
the eviction itself (because the evict thread is not able to keep up).
This can result in massive lock contention.
It isn't necessary to hold the lock, because if we make the wrong choice
occasionally, nothing bad will happen.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>