client: Make truncation work properly

client: Make truncation work properly
The previous if block didn't work because inode->size was usually
changed well before handle_cap_trunc was ever invoked, so it never
did the truncation in the objectcacher! This was okay if you just truncated
a file and then closed it, but if you wrote a file, truncated part of it out,
and then wrote past the (new) end you would get reads that returned
the previously-truncated data out of what should have been a hole.

Now, we do the actual objectcacher truncation in update_inode_file_bits,
because all methods of truncation will move through there and this maintains
proper ordering.