btrfs: move btrfs_truncate_block out of trans handle

Since we do a delalloc reserve in btrfs_truncate_block we can deadlock
with freeze.  If somebody else is trying to allocate metadata for this
inode and it gets stuck in start_delalloc_inodes because of freeze we
will deadlock.  Be safe and move this outside of a trans handle.  This
also has a side-effect of making sure that we're not leaving stale data
behind in the other_encoding or encryption case.  Not an issue now since
nobody uses it, but it would be a problem in the future.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>