7910 l2arc_write_buffers() may write beyond target_sz

illumos/illumos-gate@16a7e5ac116c85d965007a5f201104b564e82210
https://github.com/illumos/illumos-gate/commit/16a7e5ac116c85d965007a5f201104b564e82210

https://www.illumos.org/issues/7910

It seems that the change in issue #6950 resurrected the problem that was
earlier fixed by the change in issue #5219.
Please also see the following FreeBSD bug report: https://bugs.freebsd.org/
bugzilla/show_bug.cgi?id=216178

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>