3120 zinject hangs in zfsdev_ioctl() due to uninitialized zc
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

Fixes zinject panic.
Resolves https://github.com/zfs-osx/zfs/issues/113

(cherry picked from commit f4c46b1eda9212fd32ba197043d52239ef5c0a7f)
Signed-off-by: ilovezfs <ilovezfs@icloud.com>
Prevent infinite loop in zinject.

Since OS X can return 0 from ioctl even when there was a zfs error,
zinject needs to check zc_ioc_error and set errno, which is handled
for us if we use zfs_ioctl instead of ioctl.