Branch b1_6

Fix up error message for the case where a client hits a bogus checksum
error.  This only happens during mmap IO, and can be seen when the
client-side checksum matches the new checksum found on the server,
or has changed again since the message was originally sent.

In preparation for turning checksumming on by default.
b=11742
i=scjody
i=adilger (behlendorf original patch)