MFC r289295: 5219 l2arc_write_buffers() may write beyond target_sz

MFC r289295: 5219 l2arc_write_buffers() may write beyond target_sz

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov@gmail.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Steven Hartland <steven.hartland@multiplay.co.uk>
Reviewed by: Justin Gibbs <gibbs@FreeBSD.org>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Andriy Gapon <avg@freebsd.org>

illumos/illumos-gate@d7d9a6d919f92d74ea0510a53f8441396048e800

git-svn-id: svn://svn.freebsd.org/base/stable/10@290749 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f