engines: rename option specific pad from 'td' to 'pad'

Except the net engine, they all just use it as padding to avoid
having a zero offset member. The latter makes the parser think
the offset isn't set, so not valid.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/cpu.c b/engines/cpu.c
index 85598ef..7e4d737 100644
--- a/engines/cpu.c
+++ b/engines/cpu.c
@@ -8,7 +8,7 @@
 #include "../fio.h"
 
 struct cpu_options {
-	struct thread_data *td;
+	void *pad;
 	unsigned int cpuload;
 	unsigned int cpucycle;
 	unsigned int exit_io_done;
diff --git a/engines/e4defrag.c b/engines/e4defrag.c
index 3599ab8..6854070 100644
--- a/engines/e4defrag.c
+++ b/engines/e4defrag.c
@@ -36,7 +36,7 @@
 };
 
 struct e4defrag_options {
-	struct thread_data *td;
+	void *pad;
 	unsigned int inplace;
 	char * donor_name;
 };
diff --git a/engines/gfapi.h b/engines/gfapi.h
index 0da471c..1028431 100644
--- a/engines/gfapi.h
+++ b/engines/gfapi.h
@@ -2,7 +2,7 @@
 #include "../fio.h"
 
 struct gf_options {
-	struct thread_data *td;
+	void *pad;
 	char *gf_vol;
 	char *gf_brick;
 };
diff --git a/engines/libaio.c b/engines/libaio.c
index f7e2f89..d4f4830 100644
--- a/engines/libaio.c
+++ b/engines/libaio.c
@@ -37,7 +37,7 @@
 };
 
 struct libaio_options {
-	struct thread_data *td;
+	void *pad;
 	unsigned int userspace_reap;
 };
 
diff --git a/engines/rbd.c b/engines/rbd.c
index f0a0e5b..3688577 100644
--- a/engines/rbd.c
+++ b/engines/rbd.c
@@ -25,7 +25,7 @@
 };
 
 struct rbd_options {
-	struct thread_data *td;
+	void *pad;
 	char *rbd_name;
 	char *pool_name;
 	char *client_name;