f2fs: fix incorrect parsing with option string

Previously 'background_gc={on***,off***}' is being parsed as correct option,
with this patch we cloud fix the trivial bug in mount process.

Change log from v1:
 o need to check length of parameter suggested by Jaegeuk Kim.
Signed-off-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/super.c

@@ -258,9 +258,9 @@ static int parse_options(struct super_block *sb, char *options)
 
 			if (!name)
 				return -ENOMEM;
-			if (!strncmp(name, "on", 2))
+			if (strlen(name) == 2 && !strncmp(name, "on", 2))
 				set_opt(sbi, BG_GC);
-			else if (!strncmp(name, "off", 3))
+			else if (strlen(name) == 3 && !strncmp(name, "off", 3))
 				clear_opt(sbi, BG_GC);
 			else {
 				kfree(name);