libzfs_sendrecv: Style pass on dump_filesystem

• Add high level comments.
• Eliminate unnecessarily void arg.
• Avoid unnecessary line wrapping.
• Initialize sdd fields with the correct types.
• Remove extra whitespace.
• Refactor replication checks for clarity.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12967