backend: respect return value of init_io_u_buffers

backend: respect return value of init_io_u_buffers

When workloads require large buffer for I/O, fio fails to allocate I/O
buffer but does not report meaningful error message. It just accesses
to null pointer and fail with signal 11. This symptom is observed with
the command line below:

$ fio --name=job --filename=/tmp/fio --rw=write --bs=1g --size=1g \
      --iodepth=128 --ioengine=libaio

The I/O buffer allocation is done in function init_io_u_buffers. The
allocation failure is not reported because return value of the function
is ignored. Check the return value and report to the higher layer.

Fixes: 71e6e5a2fd5c ("iolog replay: Realloc io_u buffers to adapt to operation size.")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20221201024425.2340442-1-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>