call-stub: remove fall-through in ftruncate

Original-author: Pranith Kumar K <pkarampu@redhat.com>

Change-Id: I7c8ed3e3e4ddc558ff061f2e1407fb43095e67f4
BUG: 913544
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/5827
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>