btrfs: make subvol creation/deletion killable in the early stages

The subvolume ioctls block on the parent directory mutex that can be
held by other concurrent snapshot activity for a long time. Give the
user at least some chance to get out of this situation by allowing
to send a kill signal.
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ioctl.c

@@ -723,7 +723,9 @@ static noinline int btrfs_mksubvol(struct path *parent,
 	struct dentry *dentry;
 	int error;
 
-	mutex_lock_nested(&dir->i_mutex, I_MUTEX_PARENT);
+	error = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
+	if (error == -EINTR)
+		return error;
 
 	dentry = lookup_one_len(name, parent->dentry, namelen);
 	error = PTR_ERR(dentry);
@@ -2086,7 +2088,9 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	if (err)
 		goto out;
 
-	mutex_lock_nested(&dir->i_mutex, I_MUTEX_PARENT);
+	err = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
+	if (err == -EINTR)
+		goto out;
 	dentry = lookup_one_len(vol_args->name, parent, namelen);
 	if (IS_ERR(dentry)) {
 		err = PTR_ERR(dentry);