svcrdma: Fix pcl_for_each_segment for empty chunks

When a parsed chunk list contains a chunk whose ch_segcount is zero,
pcl_for_each_segment computes its inclusive upper bound as
&chunk->ch_segments[ch_segcount - 1]. ch_segcount is u32, so the
subtraction wraps to 0xFFFFFFFF and the bound lands far past the
ch_segments flex array. The loop body then walks unrelated memory at
sizeof(struct svc_rdma_segment) stride until it faults.

A zero-segcount chunk is reachable from the wire:
xdr_check_write_chunk() only rejects segcount values greater than
rc_maxpages, and pcl_alloc_write() links a freshly allocated chunk
onto rc_write_pcl/rc_reply_pcl before its segment-fill loop runs,
so a Write or Reply chunk advertising zero segments leaves
ch_segcount == 0 on the list. When the transport has negotiated
Send-With-Invalidate, svc_rdma_get_inv_rkey() iterates all four
PCLs with pcl_for_each_segment and dereferences segment->rs_handle
on each iteration, turning the underflow into an out-of-bounds read
and a general protection fault.

    xdr_check_write_list / xdr_check_reply_chunk
      pcl_alloc_write()
        chunk = pcl_alloc_chunk(...)  /* ch_segcount = 0 */
        list_add_tail(&chunk->ch_list, &pcl->cl_chunks)
        /* fill loop iterates zero times for wire segcount 0 */

    svc_rdma_get_inv_rkey()
      pcl_for_each_chunk(rc_write_pcl)
        pcl_for_each_segment(segment, chunk)
          pos <= &ch_segments[0u - 1u]  /* 0xFFFFFFFF */
          segment->rs_handle            /* OOB read -> GPF */

Fix by switching the macro to a half-open upper bound that uses
ch_segcount directly. For ch_segcount == 0 the loop start equals the
loop end and the body is skipped; for ch_segcount > 0 the iteration
range is unchanged. All six existing call sites in
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c and
net/sunrpc/xprtrdma/svc_rdma_rw.c remain correct under the new bound,
so no caller changes are needed.

Fixes: 78147ca8b4 ("svcrdma: Add a "parsed chunk list" data structure")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-rpc-kernel-bugs-v1-4-e251306ccca9@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc_rdma_pcl.h

@@ -97,7 +97,7 @@ pcl_next_chunk(const struct svc_rdma_pcl *pcl, struct svc_rdma_chunk *chunk)
  */
 #define pcl_for_each_segment(pos, chunk) \
 	for (pos = &(chunk)->ch_segments[0]; \
-	     pos <= &(chunk)->ch_segments[(chunk)->ch_segcount - 1]; \
+	     pos < &(chunk)->ch_segments[(chunk)->ch_segcount]; \
 	     pos++)
 
 /**