f2fs-tools: discard is default but not set in config

flash devices support discard therefore discard is default but not set
in config

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/lib/libf2fs.c b/lib/libf2fs.c
index 988c854..3984697 100644
--- a/lib/libf2fs.c
+++ b/lib/libf2fs.c
@@ -371,6 +371,7 @@
 	c->heap = 1;
 	c->vol_label = "";
 	c->device_name = NULL;
+	c->trim = 1;
 }
 
 static int is_mounted(const char *mpt, const char *device)