sunrpc: don't pass on-stack memory to sg_set_buf

As of ac4e97ab

 "scatterlist: sg_set_buf() argument must be in linear
mapping", sg_set_buf hits a BUG when make_checksum_v2->xdr_process_buf,
among other callers, passes it memory on the stack.

We only need a scatterlist to pass this to the crypto code, and it seems
like overkill to require kmalloc'd memory just to encrypt a few bytes,
but for now this seems the best fix.

Many of these callers are in the NFS write paths, so we allocate with
GFP_NOFS.  It might be possible to do without allocations here entirely,
but that would probably be a bigger project.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>