set_global_var_parse_kv() should pass the pointer from strdup()

A comment says that the caller should free k_out, but the pointer passed
via k_out is not the same pointer we received from strdup(). Instead,
it is a pointer into the region we received from strdup(). The free
function should always be called with the original pointer, so this is
likely a bug.

We solve this by calling strdup() a second time and then freeing the
original pointer.

Coverity reported this as a memory leak.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13867