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: Ied062dc21ffa0a774244245e77f1fff0578f318a
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>