Remove superfluous statement

Clang's static analyzer reported that the value assigned to pcksum is
never used. That is because we initialize both zc and pcksum to {{ 0 }}
and then do pcksum = zc;. That is fairly pointless. However, it has
the effect of generating a false positive in Clang's static analyzer.
Since noise from false positives can obscure real issues, we fix it
anyway.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2330