libzfs_sendrecv: Use fnv* to verify nvlist/nvpair*

Use verified variants of nvlist/nvpair functions where applicable.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11460