rpc,glusterd: Set ret to 0 after call to rpc_clnt_submit()

This is to guard against a double STACK_DESTROY in the event that
rpc_clnt_submit() failed and returned -1 anytime after sending the
request over the wire. in which case the stack could be destroyed
twice: once in the callback function of the request and once in
the error codepath of some of the callers of glusterd_submit_request().
This bug was introduced in http://review.gluster.org/#/c/8257/

Change-Id: I18a345db6eafc62ba312743201ced7d3f8697622
BUG: 1116243
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/8301
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>