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>

include/linux/libnvdimm.h

@@ -27,7 +27,7 @@ enum {
 	/* need to set a limit somewhere, but yes, this is likely overkill */
 	ND_IOCTL_MAX_BUFLEN = SZ_4M,
 	ND_CMD_MAX_ELEM = 5,
-	ND_CMD_MAX_ENVELOPE = 16,
+	ND_CMD_MAX_ENVELOPE = 256,
 	ND_MAX_MAPPINGS = 32,
 
 	/* region flag indicating to direct-map persistent memory by default */