libnvdimm: increase max envelope size for ioctl

nd_ioctl() must first read in the fixed sized portion of an ioctl so
that it can then determine the size of the variable part.

Prepare for ND_CMD_CALL calls which have larger fixed portion
envelope.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>