xprtrdma: Sanitize the reply credit grant after parsing

xprtrdma: Sanitize the reply credit grant after parsing

The out_norqst exit in rpcrdma_reply_handler() branches away before
the credit clamp, so a reply that matches no pending request reaches
out_post carrying the raw credit value parsed from the wire.
rpcrdma_post_recvs() does not bound its @needed argument: the refill
loop allocates and chains Receive WRs until the count is satisfied or
allocation fails. A peer that sends a well-formed reply carrying an
unknown XID and an inflated credit grant therefore drives rep
allocation and Receive posting past re_max_requests on every such
reply.

Move the clamp to immediately after the credit field is parsed,
ahead of the first branch that can reach out_post, so every later
consumer sees a sanitized value. The cwnd update stays on the
matched-request path.

Fixes: 704f3f640f72 ("xprtrdma: Post receive buffers after RPC completion")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>