f2fs: fix f2fs_show_options to show nogc_merge mount option

Commit 5911d2d1 ("f2fs: introduce gc_merge mount option") forgot
to show nogc_merge option, let's fix it.
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1902,6 +1902,8 @@ static int f2fs_show_options(struct seq_file *seq, struct dentry *root)
 
 	if (test_opt(sbi, GC_MERGE))
 		seq_puts(seq, ",gc_merge");
+	else
+		seq_puts(seq, ",nogc_merge");
 
 	if (test_opt(sbi, DISABLE_ROLL_FORWARD))
 		seq_puts(seq, ",disable_roll_forward");