SUNRPC: Remove XDRBUF_SPARSE_PAGES flag in gss_proxy upcall

There's no need to defer allocation of pages for the receive buffer.

- This upcall is quite infrequent
- gssp_alloc_receive_pages() can allocate the pages with GFP_KERNEL,
  unlike the transport
- gssp_alloc_receive_pages() knows exactly how many pages are needed

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Olga Kornievskaia <kolga@netapp.com>