OpenZFS 7304 - zfs filesystem/snapshot counts should be read-only

Authored by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7304
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/007a6c1
Closes #5624