Btrfs: fix pages truncation in btrfs_ioctl_clone()

It's a bug in commit f81c9cdc
(Btrfs: truncate pages from clone ioctl target range)

We should pass the dest range to the truncate function, but not the
src range.

Also move the function before locking extent state.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>