performance/md-cache: add force-readdirp flag to make readdirp configurable

md-cache currently transforms all readdir fops into readdirp fops.
This patch creates the 'force-readdirp' configuration flag to
provide control over this behavior. force-readdirp is enabled by
default to maintain current default behavior.

BUG: 903175
Change-Id: Idd70926dec7c271204bdfb11fb052e56d0a39420
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/4440
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>