posix: Fix readv FOP

Suggested by Anand Avati in BD xlator code review.

Change-Id: I31c353a26dfdeb3d0023c3f7e03ed25461d13c16
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
BUG: 837495
Reviewed-on: http://review.gluster.org/6077
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>