nfsd: Fix pNFS handling of Deallocate

For a pNFS server configuration, an NFSv4.2 Deallocate operation
is proxied to the DS(s). The code that parsed the reply for the
proxy RPC is broken and did not process the pre-operation attributes.

This patch fixes this problem.

This bug would only affect pNFS servers built from recent main/FreeBSD14
sources.