cluster/afr: Fix mem-leak

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

Change-Id: Ib1548f2df97688859f2cace44b93b3b733297c36
BUG: 1128801
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8457
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>