btrfs: transaction_kthread() is not freezable

transaction_kthread() is calling try_to_freeze(), but that's just an
expeinsive no-op given the fact that the thread is not marked freezable.

After removing this, disk-io.c is now independent on freezer API.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: David Sterba <dsterba@suse.com>