svcrdma: Factor out WR chain linking into helper

svcrdma: Factor out WR chain linking into helper

svc_rdma_prepare_write_chunk() and svc_rdma_prepare_reply_chunk()
contain identical code for linking RDMA R/W work requests onto a
Send context's WR chain. This duplication increases maintenance
burden and risks divergent bug fixes.

Introduce svc_rdma_cc_link_wrs() to consolidate the WR chain
linking logic. The helper walks the chunk context's rwctxts list,
chains each WR via rdma_rw_ctx_wrs(), and updates the Send
context's chain head and SQE count. Completion signaling is
requested only for the tail WR (posted first).

No functional change.

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