btrfs-progs: introduce a proper structure on which cli will call register-device...

btrfs-progs: introduce a proper structure on which cli will call register-device ioctl

As of now commands mentioned below (with in [..]) are calling call register-device
ioctl BTRFS_IOC_SCAN_DEV for all the devices in the system.
Some issues with it:
 BTRFS_IOC_SCAN_DEV: ioctl is a write operation, we don't want command like
 btrfs-debug-tree threads to do that..
   eg:
   ----
   $ cat /proc/fs/btrfs/devlist  | egrep fsid | wc -l
   0
   $ btrfs-debug-tree /dev/sde  (num_device > 1)
   $ cat /proc/fs/btrfs/devlist  | egrep fsid | wc -l
   5
   ----

 btrfs_scan_fs_devices() ends up calling this ioctl only when num_device > 1.
 That's inconsistency with in feature/bug.

 We don't have to register _all_ the btrfs devices (again) in the system
 without user consent.

Why its inconsistent:
 function btrfs_scan_fs_devices() calls btrfs_scan_lblkid only when
 num_devices is > 1, which in turn calls BTRFS_IOC_SCAN_DEV ioctl, if
 conditions are met.

 But main issue is we have too many consumers of btrfs_scan_fs_devices()
 the names below with in [] is the cli leading to this function.

 open_ctree_broken()  [btrfs-find-root]
 recover_prepare()    [btrfs rescue super-recover]
 __open_ctree_fd
 (updates always except when flag OPEN_CTREE_RECOVER_SUPER is set and
 flag OPEN_CTREE_RECOVER_SUPER is set only by 'btrfs rescue super-
 recover' but still this thread sneaks through the open_ctree function
 to call register-device-ioctl as show below).
	open_ctree_fs_info
		[btrfs-debug-tree]
		[btrfs-image -r]
		[btrfs check]
		open_fs
			[btrfs restore]
		open_ctree
			[calc-size]
			[btrfs-corrupt-block]
			[btrfs-image] (create)
			[btrfs-map-logical]
			[btrfs-select-super]
			[btrfstune]
			[btrfs-zero-log]
			[tester]
			[mkfs]
			[quick-test.c]
			[btrfs label set unmounted]
			[btrfs get label unmounted]
			[btrfs rescue super-recover]

	open_ctree_fd
		[btrfs-convert]

Fix:
 In an effort to make register-device consistent, all calls to
 btrfs_scan_fs_devices() will have 5th parameter set to 0. that means
 we don't need 5th parameter at all. And with this function not calling
 the register ioctl at all, finally we will have following two cli to call
 the ioctl BTRFS_IOC_SCAN_DEV.
    btrfs dev scan and
    mkfs.btrfs
 Threads needing to update kernel about a device would have to use
 btrfs_register_one_device() separately.

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