btrfs: split btrfs_setxattr calls regarding transaction

When the caller has already created the transaction handle,
btrfs_setxattr() will use it. Also adds assert in btrfs_setxattr().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>