f2fs: return a default value for non-void function

This patch resolves a build warning reported by kbuild test robot.

"
fs/f2fs/segment.c: In function '__get_segment_type':
fs/f2fs/segment.c:806:1: warning: control reaches end of non-void
function [-Wreturn-type]
"

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 50240d2..cf0ffb8 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -302,7 +302,7 @@
 		case Opt_active_logs:
 			if (args->from && match_int(args, &arg))
 				return -EINVAL;
-			if (arg != 2 && arg != 4 && arg != 6)
+			if (arg != 2 && arg != 4 && arg != NR_CURSEG_TYPE)
 				return -EINVAL;
 			sbi->active_logs = arg;
 			break;