gfapi: Fix iobuf leaks in gfapi

In glfs_readv, if syncop_readv() fails with return value <= 0
the iobref was not being unrefd which would cause iobuf leaks.

Change-Id: I9850ae149e53cf75ba26f8b5f4c5446cce4b5991
BUG: 1018176
Signed-off-by: Poornima <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/6074
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>