MFC r322222: MFV r322221: 7910 l2arc_write_buffers() may write beyond target_sz

FreeBD note: the essence of this change was committed to FreeBSD in
r314274. This commit catches up with differences between what was
committed to FreeBSD and what was committed to OpenZFS, mainly more
logical variable names.

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>