xprtrdma: Scale receive batch size with credit window

The fixed RPCRDMA_MAX_RECV_BATCH of 7 results in frequent
small ib_post_recv batches during high-rate workloads. With
a 128-slot credit window, receives are reposted every 7th
completion, each batch incurring atomic serialization and a
doorbell write.

Replace the fixed batch constant with a per-endpoint value
scaled to 25% of the negotiated credit window. For a typical
128-credit connection this raises the batch from 7 to 32,
reducing doorbell frequency by roughly 4x and amortizing the
per-batch atomic and MMIO costs over a larger group of
receive WRs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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