libzfs_sendrecv: Use fnv* to verify nvlist/nvpair*

Use verified variants of nvlist/nvpair functions where applicable.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Matthew Ahrens <[email protected]>
Signed-off-by: Ryan Moeller <[email protected]>
Closes #11460

Author: Ryan Moeller