Fix gcc 10.1 stringop-truncation error

As we do not expect the destination of these strncpy calls to be NULL
terminated, substitute them with memcpy.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10346