Fix unreachable code in zstreamdump

82226e4f44baa3f7c3101caaaf941927aa318e46 was intended to prevent a
warning from being printed in situations where it was inappropriate, but
accidentally disabled it entirely by setting featureflags in the wrong
case statement.

Coverity reported this as dead code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13946