crfsdctl: fix unaligned access fault in pr_hello

[Zach Brown]

Alex Chiang wrote:
> Hi Zach,
> 
> Not sure how much we care about this, but I get at least one
> unaligned access warning while running crfsdctl snoop:

We certainly care about this.  Thanks for testing on a platform which
whines about this ;).

> Here's a patch that fixes it -- there are definitely more clever
> ways to do this

How about just slapping __attribute__((packed)) on struct crfs_msg_hello?

- z