A few fixes of callback typecasting (for the upcoming ClangCFI)

• zio: avoid callback typecasting
• zil: avoid zil_itxg_clean() callback typecasting
• zpl: decouple zpl_readpage() into two separate callbacks
• nvpair: explicitly declare callbacks for xdr_array()
• linux/zfs_nvops: don't use external iput() as a callback
• zcp_synctask: don't use fnvlist_free() as a callback
• zvol: don't use ops->zv_free() as a callback for taskq_dispatch()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Closes #12260