rpcsvc: fix dangerous setting of pointer on free'd structure

The current code is setting @req->hdr_iobuf = NULL _after_ calling
actor_fn() on @req. Calling actor_fn() takes away all guarantees
of whether @req is still a valid object or destroyed.

Unfortunately most of the times the object is allocated from mem-pool
an a mem_put() still keeps the arena allocated (no crash). However
once the mem-pool is full and allocation falls back to malloc()/free()
the code actually becomes dangerous. This resulted in random crashes
when the system load is high (when there were sufficient outstanding
calls that @rpc pool got full)

Change-Id: I4398c717aa0e2c5f06733212b64dd79e7b2a4136
BUG: 884452
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4990
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>