Fix zfsctl_snapshot_{,un}mount() issues

Fix use after free in zfsctl_snapshot_unmount(). Use /usr/bin/env
instead of /bin/sh to fix a shell code injection flaw and allow use
with grsecurity.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Stian Ellingsen <stian@plaimi.net>
Closes #5250
Closes #4377