SUNRPC: Track consumed rq_pages entries

The rq_pages array holds pages allocated for incoming RPC requests.
Two transport receive paths NULL entries in rq_pages to prevent
svc_rqst_release_pages() from freeing pages that the transport has
taken ownership of:

- svc_tcp_save_pages() moves partial request data pages to
  svsk->sk_pages during multi-fragment TCP reassembly.

- svc_rdma_clear_rqst_pages() moves request data pages to
  head->rc_pages because they are targets of active RDMA Read WRs.

A new rq_pages_nfree field in struct svc_rqst records how many
entries were NULLed. svc_alloc_arg() uses it to refill only those
entries rather than scanning the full rq_pages array. In steady
state, the transport NULLs a handful of entries per RPC, so the
allocator visits only those entries instead of the full ~259 slots
(for 1MB messages).

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

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