Fix 2 small bugs with cached dsl_scan_phys_t

This patch corrects 2 small bugs where scn->scn_phys_cached was
not properly updated to match the primary copy when it needed to
be. The first resulted in the pause state not being properly
updated and the second resulted in the cached version being
completely zeroed even if the primary was not.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8010