Btrfs: remove mnt_want_write call in btrfs_mksubvol

We got a recursive lock in mksubvol because the caller already held
a lock. I think we got into this due to a merge error. Commit a874a63e
removed the mnt_want_write call from btrfs_mksubvol and added a
replacement call to mnt_want_write_file in btrfs_ioctl_snap_create_transid.
Commit e7848683 however tried to move all calls to mnt_want_write above
i_mutex. So somewhere while merging this, it got mixed up. The
solution is to remove the mnt_want_write call completely from
mksubvol.
Reported-by: David Sterba <dave@jikos.cz>
Signed-off-by: Alexander Block <ablock84@googlemail.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ioctl.c

@@ -664,10 +664,6 @@ static noinline int btrfs_mksubvol(struct path *parent,
 	struct dentry *dentry;
 	int error;
 
-	error = mnt_want_write(parent->mnt);
-	if (error)
-		return error;
-
 	mutex_lock_nested(&dir->i_mutex, I_MUTEX_PARENT);
 
 	dentry = lookup_one_len(name, parent->dentry, namelen);
@@ -703,7 +699,6 @@ static noinline int btrfs_mksubvol(struct path *parent,
 	dput(dentry);
 out_unlock:
 	mutex_unlock(&dir->i_mutex);
-	mnt_drop_write(parent->mnt);
 	return error;
 }