cluster/dht: Make sure gf_defrag_migrate_data is not optimized

Problem:
Whenever there syncop_xxx() is used inside a synctask and gcc
optimizes it when compiled with -O2 there is a problem where
'errno' would not work as expected.

Fix:
Until http://review.gluster.com/6475 is reviewed and merged
we are making sure the function is not going to be optimized.

Change-Id: I504c18c8a7789f0c776a56f0aa60db3618b21601
BUG: 1040356
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/6481
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>