rpc: before freeing the volume options object, delete it from the list

* Suppose there is an xlator option which is considered by the xlator
  only if the source was built with debug mode enabled (the only example
  in the current code base is run-with-valgrind option for glusterd), then
  giving that option would make the process crash if the source was not
  built with debug mode enabled.

  Reason:
  In rpc, after getting the options symbol dynamically, it was stored in the
  newly allocated volume options structure and the structure's list head was
  added to the xlator's volume_options list. But while freeing the structure
  the list was not deleted. Thus when the list was traversed, already freed
  structure was accessed leading to segfault.

Change-Id: I3e9e51dd2099e34b206199eae7ba44d9d88a86ad
BUG: 922877
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4687
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>