MFC r305205: MFV r302657:

MFC r305205: MFV r302657:
4521 zfstest is trying to execute evil "zfs unmount -a"

illumos/illumos-gate@8808ac5dae118369991f158b6ab736cb2691ecde
https://github.com/illumos/illumos-gate/commit/8808ac5dae118369991f158b6ab736cb2
691ecde

https://www.illumos.org/issues/4521
  zfstest is trying to execute evil "zfs unmount -a", which fails (fortunately,
  as it would otherwise leave me with my ~ missing):
  03:44:11.86 cannot unmount '/export/home/yuri': Device busy cannot unmount '/
  export/home': Device busy
  03:44:11.86 ERROR: /usr/sbin/zfs unmount -a exited 1
  This affects, at least, zfs_mount_009_neg and zfs_mount_all_001_pos, both
  failing on that step. The pool containing the /export/home hierarchy is
  included in KEEP variable, but it doesn't seem to affect anything here.

Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>

git-svn-id: svn://svn.freebsd.org/base/stable/10@307120 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f