fuse: use postbuf in truncate_cbk for attributes to return
fuse: use postbuf in truncate_cbk for attributes to return

this causes a problem where inodes even after truncate show old
file size for a short time (till attributes are expired in fuse kernel module)

Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 146 (Add setattr FOP)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=146

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster