Btrfs-progs: detect when scrub is started twice

Btrfs-progs: detect when scrub is started twice

Check whether any involved device is already busy running a
scrub. This would cause damaged status messages and the state
"aborted" without the explanation that a scrub was already
running. Therefore check it first, prevent it and give some
feedback to the user if scrub is already running.
Note that if scrub is started with a block device as the
parameter, only that particular block device is checked. It
is a normal mode of operation to start scrub on multiple
single devices, there is no reason to prevent this.

Here is an example:
/mnt2 is the mountpoint of a filesystem.
/dev/sdk and /dev/sdl are the block devices for that filesystem.

case 1:
btrfs scrub start /mnt2
btrfs scrub start /mnt2
-> complain

case 1:
btrfs scrub start /dev/sdk
btrfs scrub start /dev/sdk
-> complain

case 3:
btrfs scrub start /dev/sdk
btrfs scrub start /dev/sdl
-> don't complain

case 4:
btrfs scrub start /dev/sdk
btrfs scrub start /mnt2
-> complain

case 5:
btrfs scrub start /mnt2
btrfs scrub start /dev/sdk
-> complain if the scrub on /dev/sdk is still running.
-> don't complain if the scrub on /dev/sdk is finished, the
   status messages will be fine.

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>