svcrdma: use bvec-based RDMA read/write API

svcrdma: use bvec-based RDMA read/write API

Convert svcrdma to the bvec-based RDMA API introduced earlier in
this series.

The bvec-based RDMA API eliminates the intermediate scatterlist
conversion step, allowing direct DMA mapping from bio_vec arrays.
This simplifies the svc_rdma_rw_ctxt structure by removing the
chained SG table management.

The structure retains an inline array approach similar to the
previous scatterlist implementation: an inline bvec array sized
to max_send_sge handles most I/O operations without additional
allocation. Larger requests fall back to dynamic allocation.
This preserves the allocation-free fast path for typical NFS
operations while supporting arbitrarily large transfers.

The bvec API handles all device types internally, including iWARP
devices which require memory registration. No explicit fallback
path is needed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260128005400.25147-6-cel@kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Leon Romanovsky <leon@kernel.org>