svcrdma: Use contiguous pages for RDMA Read sink buffers

svc_rdma_build_read_segment() constructs RDMA Read sink
buffers by consuming pages one-at-a-time from rq_pages[]
and building one bvec per page. A 64KB NFS READ payload
produces 16 separate bvecs, 16 DMA mappings, and
potentially multiple RDMA Read WRs (on platforms with
4KB pages).

A single higher-order allocation followed by split_page()
yields physically contiguous memory while preserving
per-page refcounts. A single bvec spanning the contiguous
range causes rdma_rw_ctx_init_bvec() to take the
rdma_rw_init_single_wr_bvec() fast path: one DMA mapping,
one SGE, one WR.

The split sub-pages replace the original rq_pages[] entries,
so all downstream page tracking, completion handling, and
xdr_buf assembly remain unchanged.

Allocation uses __GFP_NORETRY | __GFP_NOWARN and falls back
through decreasing orders. If even order-1 fails, the
existing per-page path handles the segment.

When nr_pages is not a power of two, get_order() rounds up
and the allocation yields more pages than needed. The extra
split pages replace existing rq_pages[] entries (freed via
put_page() first), so there is no net increase in per-
request page consumption. Successive segments reuse the
same padding slots, preventing accumulation. The
rq_maxpages guard rejects any allocation that would
overrun the array, falling back to the per-page path.
Under memory pressure, __GFP_NORETRY causes the higher-
order allocation to fail without stalling.

The contiguous path is attempted when the segment starts
page-aligned (rc_pageoff == 0) and spans at least two
pages. NFS WRITE segments carry application-modified byte
ranges of arbitrary length, so the optimization is not
restricted to power-of-two page counts.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

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