NFSD: Use svcxdr_encode_opaque_pages() in nfsd4_encode_splice_read()

NFSD: Use svcxdr_encode_opaque_pages() in nfsd4_encode_splice_read()

Commit 15b23ef5d348 ("nfsd4: fix corruption of NFSv4 read data")
encountered exactly the same issue: after a splice read, a
filesystem-owned page is left in rq_pages[]; the symptoms are the
same as described there.

If the computed number of pages in nfsd4_encode_splice_read() is not
exactly the same as the actual number of pages that were consumed by
nfsd_splice_actor() (say, because of a bug) then hilarity ensues.

Instead of recomputing the page offset based on the size of the
payload, use rq_next_page, which is already properly updated by
nfsd_splice_actor(), to cause svc_rqst_release_pages() to operate
correctly in every instance.

This is a defensive change since we believe that after commit
27c934dd8832 ("nfsd: don't replace page in rq_pages if it's a
continuation of last page") has been applied, there are no known
opportunities for nfsd_splice_actor() to screw up. So I'm not
marking it for stable backport.

Reported-by: Andy Zlotek <andy.zlotek@oracle.com>
Suggested-by: Calum Mackay <calum.mackay@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>