snapuserd: Fallback to synchronous I/O if any errors observed during

async merge.

If there are any I/O errors during async merge, we will
retry the I/O in synchronous I/O path. For this to happen,
we have to reset the iterator so that we replay the blocks
which were partially completed during async merge. Furthermore,
we will disable the async merge and continue to do the I/O
in synchronous path.

Additionally, cut down the queue depth to 8 so that
it will decerease the number of the async offload. We don't
want to have a big queue depth with async offload.

Bug: 220991038
Test: Instrument the code to fail the Async I/O's
randomly and make sure merge is completed. Instrumentation
was done both on readahead and merge code path.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I0db6d0f46054ca5b8423201a598c726b2c3d21ac