iolog: don't copy log samples for compression

iolog: don't copy log samples for compression

For larger settings of log_compression, the allocation and copying of
the log will adversely affect performance and logged latencies. Instead
just reference the log in the compression code, while resetting the
current log to a smaller size. We pay a slightly higher cost in memory
until compression has completd, but it saves us from allocating and
copying tens (or hundreds) of megabytes inline.

Signed-off-by: Jens Axboe <axboe@fb.com>