svcrdma: Fix NFS server crash triggered by 1MB NFS WRITE

Now that the NFS server advertises a maximum payload size of 1MB
for RPC/RDMA again, it crashes in svc_process_common() when NFS
client sends a 1MB NFS WRITE on an NFS/RDMA mount.

The server has set up a 259 element array of struct page pointers
in rq_pages[] for each incoming request. The last element of the
array is NULL.

When an incoming request has been completely received,
rdma_read_complete() attempts to set the starting page of the
incoming page vector:

  rqstp->rq_arg.pages = &rqstp->rq_pages[head->hdr_count];

and the page to use for the reply:

  rqstp->rq_respages = &rqstp->rq_arg.pages[page_no];

But the value of page_no has already accounted for head->hdr_count.
Thus rq_respages now points past the end of the incoming pages.

For NFS WRITE operations smaller than the maximum, this is harmless.
But when the NFS WRITE operation is as large as the server's max
payload size, rq_respages now points at the last entry in rq_pages,
which is NULL.

Fixes: cc9a903d ('svcrdma: Change maximum server payload . . .')
BugLink: https://bugzilla.linux-nfs.org/show_bug.cgi?id=270


Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Shirley Ma <shirley.ma@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>