corrupted log causes infinite loop at mount

[Eric Sandeen <sandeen@sandeen.net>]

While playing with some filesystem corruption testers, I ran into this.

http://sandeen.net/xfs.31.img.bz2

If you try to mount, it gets into xfs_buf_get_noaddr via log replay with
a len of 0, and I think this causes an infinite loop in the goto:

 try_again:
        data = kmem_alloc(malloc_len, KM_SLEEP | KM_MAYFAIL);
        if (unlikely(data == NULL))
                goto fail_free_buf;

        /* check whether alignment matches.. */
        if ((__psunsigned_t)data !=
            ((__psunsigned_t)data & ~target->bt_smask)) {
                /* .. else double the size and try again */
                kmem_free(data, malloc_len);
                malloc_len <<= 1;
                goto try_again;
        }

Up the callchain a bit there is an ASSERT that the size is > 0, but of
course that doesn't help on a non-debug kernel...

haven't had time to investigate beyond that.

-Eric