OpenZFS 6874 - rollback and receive need to reset ZPL state to what's on disk

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

When we do a clone swap (caused by "zfs rollback" or "zfs receive"), the
ZPL doesn't completely reload the state from the DMU; some values remain
cached in the zfsvfs_t.

OpenZFS-issue: https://www.illumos.org/issues/6874
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1fdcbd0
Closes #5888