Reduce the queue depth of loop devices used by the zram driver

Make the queue depth of loop devices identical to that of the underlying
storage device. This patch reduces latency by lowering the queue depth.
With this patch applied I see the following:

 # cat /sys/block/loop30/queue/nr_requests
32

Bug: 194450129
Test: Built Android images, installed these and verified that the queue depth of loop devices is 32 instead of 256.
Merged-In: Ifa16084c7df3a54d9559c2388abc4a8392ff88c6
Change-Id: Icc89e1f88d2f0ade2805999afef556b15b7ff8eb
Ignore-AOSP-First: Already in AOSP.
Signed-off-by: Bart Van Assche <bvanassche@google.com>