snapuserd: snapshot-merge optimizations

1: Remove IOSQE_ASYNC flag as this would forcefully create async job
   which is not required.

2: Cap the iou-wq threads to 1 - This will cut down the CPU usage and
   not un-necessarily spin up new threads

3: Add new ring initialization flags

4: Increase queue depth to 32

Bug: 385659495
Test: OTA on Pixel
1: Verify no more additional threads are spin up during merge
2: snapshot-merge time reduces by ~30 seconds when every block is a copy
   block.
Change-Id: Id404060ecc459ee0f95c8199bd642a07c9fb6ba6
Signed-off-by: Akilesh Kailash <akailash@google.com>