fix: drive encryption detection treated MODE SENSE success as failure

sg_modesense returns the transferred byte count (> 0) on success, but
is_ame and sg_set_key compared the result against 0/DEVICE_GOOD. is_ame
therefore always reported the drive as non-AME, and sg_set_key bailed
out before issuing SECURITY PROTOCOL OUT, so setting a data key always
failed on encrypting drives. Compare against < 0 like the other
sg_modesense callers, and normalize sg_set_key's success return.

Not verified on encrypting tape hardware; the logic follows the
documented sg_modesense return convention.

Author: matejk

src/tape_drivers/linux/sg/sg_tape.c

@@ -4630,7 +4630,9 @@ static bool is_ame(void *device)
 	unsigned char buf[TC_MP_READ_WRITE_CTRL_SIZE] = {0};
 	const int ret = sg_modesense(device, TC_MP_READ_WRITE_CTRL, TC_MP_PC_CURRENT, 0, buf, sizeof(buf));
 
-	if (ret != 0) {
+	/* sg_modesense returns the transferred byte count (> 0) on success and a
+	 * negative error code on failure. */
+	if (ret < 0) {
 		char message[100] = {0};
 		sprintf(message, "failed to get MP %02Xh (%d)", TC_MP_READ_WRITE_CTRL, ret);
 		ltfsmsg(LTFS_DEBUG, 30392D, __FUNCTION__, message);
@@ -4720,7 +4722,7 @@ int sg_set_key(void *device, const unsigned char *keyalias, const unsigned char
 
 	unsigned char buf[TC_MP_READ_WRITE_CTRL_SIZE] = {0};
 	ret = sg_modesense(device, TC_MP_READ_WRITE_CTRL, TC_MP_PC_CURRENT, 0, buf, sizeof(buf));
-	if (ret != DEVICE_GOOD)
+	if (ret < 0)  /* sg_modesense returns a byte count (> 0) on success */
 		goto out;
 
 	ltfs_u16tobe(buffer + 0, sps);
@@ -4767,8 +4769,9 @@ int sg_set_key(void *device, const unsigned char *keyalias, const unsigned char
 
 	memset(buf, 0, sizeof(buf));
 	ret = sg_modesense(device, TC_MP_READ_WRITE_CTRL, TC_MP_PC_CURRENT, 0, buf, sizeof(buf));
-	if (ret != DEVICE_GOOD)
+	if (ret < 0)  /* sg_modesense returns a byte count (> 0) on success */
 		goto out;
+	ret = DEVICE_GOOD; /* normalize the byte count to a success code */
 
 free:
 	free(buffer);