cluster/afr: Fix mem-leak

        Backport of http://review.gluster.org/8457

Problem:
local->xattr_req is already reffed with xattr_req that comes
in lookup fop. But when afr_lookup_xattr_req_prepare is called
local->xattr_req is over-written with dict_new() which leads
to ref leak on the dict which came in lookup fop

Fix:
Create local->xattr_req only when it is NULL

BUG: 1128801
Change-Id: I4a1065add7700317e0cd3d0dda0a91e12d77e340
Reviewed-on: http://review.gluster.org/8460
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>