btrfs: refuse to remount read-write after abort

It's still possible to flip the filesystem into RW mode after it's
remounted RO due to an abort. There are lots of places that check for
the superblock error bit and will not write data, but we should not let
the filesystem appear read-write.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/super.c

@@ -1340,6 +1340,12 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		if (ret)
 			goto restore;
 	} else {
+		if (test_bit(BTRFS_FS_STATE_ERROR, &root->fs_info->fs_state)) {
+			btrfs_err(fs_info,
+				"Remounting read-write after error is not allowed\n");
+			ret = -EINVAL;
+			goto restore;
+		}
 		if (fs_info->fs_devices->rw_devices == 0) {
 			ret = -EACCES;
 			goto restore;