svcrdma: Clean up chunk tracepoints

We already have trace_svcrdma_decode_rseg(), which records each
ingress Read segment. Instead of reporting those again when they
are about to be posted as RDMA Reads, let's fire one tracepoint
before posting each type of chunk.

So we'll get:

        nfsd-1998  [002]   321.666615: svcrdma_decode_rseg:  cq.id=4 cid=42 segno=0 position=0 192@0x013ca9ebfae14000:0xb0010b05
        nfsd-1998  [002]   321.666615: svcrdma_decode_rseg:  cq.id=4 cid=42 segno=1 position=0 7688@0x013ca9ebf914e000:0xb0010a05
        nfsd-1998  [002]   321.666615: svcrdma_decode_rseg:  cq.id=4 cid=42 segno=2 position=0 28@0x013ca9ebfae15000:0xb0010905
        nfsd-1998  [002]   321.666622: svcrdma_decode_rqst:  cq.id=4 cid=42 xid=0x013ca9eb vers=1 credits=128 proc=RDMA_NOMSG hdrlen=100

        nfsd-1998  [002]   321.666642: svcrdma_post_read_chunk: cq.id=3 cid=112 sqecount=3

kworker/2:1H-221   [002]   321.673949: svcrdma_wc_read:      cq.id=3 cid=112 status=SUCCESS (0/0x0)

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>