dm: md: Replace scheduling of request with direct call

This is found to be improving the performance of Random
read and write with FDE enabled. The scheduling was
introduced as part of 4.4 kernel for mq support.

Change-Id: I300750b5f7f1cff4b81f028ae938e7d2b3652bd2
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>