nilfs2: add nodiscard mount option

Nilfs has "discard" mount option which issues discard/TRIM commands to
underlying block device, but it lacks a complementary option and has
no way to disable the feature through remount.

This adds "nodiscard" option to resolve this imbalance.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

Documentation/filesystems/nilfs2.txt

@@ -77,9 +77,10 @@ norecovery		Disable recovery of the filesystem on mount.
 			This disables every write access on the device for
 			read-only mounts or snapshots.  This option will fail
 			for r/w mounts on an unclean volume.
-discard			Issue discard/TRIM commands to the underlying block
-			device when blocks are freed.  This is useful for SSD
-			devices and sparse/thinly-provisioned LUNs.
+discard			This enables/disables the use of discard/TRIM commands.
+nodiscard(*)		The discard/TRIM commands are sent to the underlying
+			block device when blocks are freed.  This is useful
+			for SSD devices and sparse/thinly-provisioned LUNs.
 
 NILFS2 usage
 ============

fs/nilfs2/super.c

@@ -604,7 +604,7 @@ static const struct export_operations nilfs_export_ops = {
 enum {
 	Opt_err_cont, Opt_err_panic, Opt_err_ro,
 	Opt_barrier, Opt_nobarrier, Opt_snapshot, Opt_order, Opt_norecovery,
-	Opt_discard, Opt_err,
+	Opt_discard, Opt_nodiscard, Opt_err,
 };
 
 static match_table_t tokens = {
@@ -617,6 +617,7 @@ static match_table_t tokens = {
 	{Opt_order, "order=%s"},
 	{Opt_norecovery, "norecovery"},
 	{Opt_discard, "discard"},
+	{Opt_nodiscard, "nodiscard"},
 	{Opt_err, NULL}
 };
 
@@ -676,6 +677,9 @@ static int parse_options(char *options, struct super_block *sb)
 		case Opt_discard:
 			nilfs_set_opt(sbi, DISCARD);
 			break;
+		case Opt_nodiscard:
+			nilfs_clear_opt(sbi, DISCARD);
+			break;
 		default:
 			printk(KERN_ERR
 			       "NILFS: Unrecognized mount option \"%s\"\n", p);