svcrdma: Improve Read chunk sanity checking

Identify malformed transport headers and unsupported chunk
combinations as early as possible.

- Reject RPC-over-RDMA messages that contain more than one Read chunk,
  since this implementation currently does not support more than one
  per RPC transaction.

- Ensure that segment lengths are not crazy.

- Remove the segment count check. With a 1KB inline threshold, the
  largest number of Read segments that can be conveyed is about 40
  (for a RDMA_NOMSG Call message). This is nowhere near
  RPCSVC_MAXPAGES. As far as I can tell, that was just a sanity
  check and does not enforce an implementation limit.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>