fd: expose __fd_ctx_del symbol

used by open-behind for deleting context in locked critical region

Change-Id: I3380152211fed6aacbb719468b720376e6ec1e7c
BUG: 846240
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4405
Tested-by: Gluster Build System <jenkins@build.gluster.com>