SUNRPC: Update svcxdr_init_decode() to call xdr_set_scratch_folio()

SUNRPC: Update svcxdr_init_decode() to call xdr_set_scratch_folio()

The only snag here is that __folio_alloc_node() doesn't handle
NUMA_NO_NODE, so I also need to update svc_pool_map_get_node() to return
numa_mem_id() instead. I arrived at this approach by  looking at what
other users of __folio_alloc_node() do for this case.

Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>