ext4: fix null handler of ioctls in no journal mode

The EXT4_IOC_GROUP_ADD and EXT4_IOC_GROUP_EXTEND ioctls should not
flush the journal in no_journal mode.  Otherwise, running resize2fs on
a mounted no_journal partition triggers the following error messages:

BUG: unable to handle kernel NULL pointer dereference at 00000014
IP: [<c039d282>] _spin_lock+0x8/0x19
*pde = 00000000 
Oops: 0002 [#1] SMP

Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ioctl.c

@@ -192,7 +192,7 @@ setversion_out:
 	case EXT4_IOC_GROUP_EXTEND: {
 		ext4_fsblk_t n_blocks_count;
 		struct super_block *sb = inode->i_sb;
-		int err, err2;
+		int err, err2=0;
 
 		if (!capable(CAP_SYS_RESOURCE))
 			return -EPERM;
@@ -205,9 +205,11 @@ setversion_out:
 			return err;
 
 		err = ext4_group_extend(sb, EXT4_SB(sb)->s_es, n_blocks_count);
-		jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
-		err2 = jbd2_journal_flush(EXT4_SB(sb)->s_journal);
-		jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
+		if (EXT4_SB(sb)->s_journal) {
+			jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
+			err2 = jbd2_journal_flush(EXT4_SB(sb)->s_journal);
+			jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
+		}
 		if (err == 0)
 			err = err2;
 		mnt_drop_write(filp->f_path.mnt);
@@ -252,7 +254,7 @@ setversion_out:
 	case EXT4_IOC_GROUP_ADD: {
 		struct ext4_new_group_data input;
 		struct super_block *sb = inode->i_sb;
-		int err, err2;
+		int err, err2=0;
 
 		if (!capable(CAP_SYS_RESOURCE))
 			return -EPERM;
@@ -266,9 +268,11 @@ setversion_out:
 			return err;
 
 		err = ext4_group_add(sb, &input);
-		jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
-		err2 = jbd2_journal_flush(EXT4_SB(sb)->s_journal);
-		jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
+		if (EXT4_SB(sb)->s_journal) {
+			jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
+			err2 = jbd2_journal_flush(EXT4_SB(sb)->s_journal);
+			jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
+		}
 		if (err == 0)
 			err = err2;
 		mnt_drop_write(filp->f_path.mnt);