NFSD: Batch release pages during splice read

Large splice reads call put_page() repeatedly. put_page() is
relatively expensive to call, so replace it with the new
svc_rqst_replace_page() helper to help amortize that cost.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: NeilBrown <neilb@suse.de>