iobuf: Added a function iobref_clear

Original-author: Venky Shankar <vshankar@redhat.com>

Change-Id: Ibf861db6c1b084b798d210962344487a1919aad2
BUG: 921942
Signed-off-by: Prashanth Pai <nullpai@gmail.com>
Reviewed-on: http://review.gluster.org/4595
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>