Btrfs: sysfs: introduce function btrfs_sysfs_add_fsid() to create sysfs fsid

We need it in a seperate function so that it can be called from the
device discovery thread as well.

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