sysfs: Register filesystems with sysfs.

sysfs: Register filesystems with sysfs.

Originally from Nikita Danliov.

- Add struct subsystem to struct file_system_type.
- Add fs_subsys_init() to explicitly call sysfs_init() very early.
- Register base fs subsystem then, too.
- Register filesystems' embedded subsystem when registered with fs core.

Resulting tree looks like:

# tree /sys/fs/
/sys/fs/
|-- bdev
|-- devpts
|-- eventpollfs
|-- ext2
|-- ext3
|-- futexfs
|-- iso9660
|-- pipefs
|-- proc
|-- ramfs
|-- rootfs
|-- sockfs
|-- sysfs
`-- tmpfs