SUNRPC: Optimize page release in svc_rdma_sendto()

SUNRPC: Optimize page release in svc_rdma_sendto()

Now that we have bulk page allocation and release APIs, it's more
efficient to use those than it is for nfsd threads to wait for send
completions. Previous patches have eliminated the calls to
wait_for_completion() and complete(), in order to avoid scheduler
overhead.

Now release pages-under-I/O in the send completion handler using
the efficient bulk release API.

I've measured a 7% reduction in cumulative CPU utilization in
svc_rdma_sendto(), svc_rdma_wc_send(), and svc_xprt_release(). In
particular, using release_pages() instead of complete() cuts the
time per svc_rdma_wc_send() call by two-thirds. This helps improve
scalability because svc_rdma_wc_send() is single-threaded per
connection.

Reviewed-by: Tom Talpey <tom@talpey.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>