Btrfs: Release uuid_mutex for shrink during device delete

Device scanning waits on the uuid_mutex, which can result in a very long
wait if dev delete is shrinking the device.

Signed-off-by: Carey Underwood <cwillu@cwillu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>