iolog replay: Realloc io_u buffers to adapt to operation size.

iolog replay: Realloc io_u buffers to adapt to operation size.

When iolog is read in chunked mode, some prefetching is done. Based on this size of buffers are determined.
Further operations may need larger buffers. If so, io_u buffers are reallocated.
Example that cause problem when read_iolog_chunked=1:
fio version 2 iolog
rbd_data.0 add
rbd_data.0 open
rbd_data.0 write 0 4096 (x20)
rbd_data.0 write 0 4096000 (x10)
rbd_data.0 close

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>